chore: initialized CHANGELOG_OLD.md with initial changes for this release branch

[williamhbaker]

Related to #22716

This creates an initial changelog file for the 2.1 release branch. Since our changelog automation is not yet capable of generating the first changelog for a release branch, I did it manually by reviewing the commit history on master and 2.0. This is the file we will actually include with the release.

To generate this new changelog, I started from the CHANGELOG_OLD file on the master branch and:

• Deleted all of the sections for previous releases: 2.0.8, 2.0.7, etc. Only the "unreleased" section was kept.
• Reviewed the release notes for the v2.0.9 release and removed all of the changes listed there.
• Starting from commit 824e76c which is where the changelog automation began and we generally stopped manually adding changelog entries, I went through all the commits in the 2.1 branch and selected anything that was a fix or a feat, + the latest flux upgrade which is a build. Any of these that were not included with the 2.0.9 release were added.

Some of the feat commits represent partial implementations of a feature (for example, many commits are related to replications). These do not have any user-facing impact and I did not include them.

For reference, this is a chronological list of commits (oldest on top) tagged as either fix or feat on the 2.1 branch since commit 824e76c, along with their categorization per the above process:

• fix: use consistent path separator in permission string representation - backported to 2.0.9 via fix: use consistent path separator in permission string representation #22517
• feat: partial support for series cardinality flux query - partial feature implementation; no changelog entry needed
• feat: deleting a bucket also deletes all associated replications - partial feature implementation; no changelog entry needed
• fix: detect noninteractive prompt when displaying warning in buildtsi - unreleased change
• fix: ensure files are closed before they are deleted or moved in deletetsm - unreleased change
• feat: multi-measurement query optimization - backported to 2.0.9 via feat: optimization for queries with predicates containing multiple measurements #22464
• fix: more expressive errors - unreleased change - NOT backported to 2.0.9 via fix: more expressive errors #22612
• feat: support for flux cardinality query - backported to 2.0.9 via feat: support for flux cardinality query #22466
• feat: allow new telegraf input plugins and update toml - unreleased change
• fix: upgrade influxql to latest version & fix predicate handling for show tag values metaqueries - backported to 2.0.9 via fix: upgrade influxql to latest version & fix predicate handling for show tag values metaqueries #22504
• feat: set X-Influxdb-Version and X-Influxdb-Build headers - backported to 2.0.9 feat: set X-Influxdb-Version and X-Influxdb-Build headers #22547
• fix: suggest setting flux Content-Type when query fails to parse as JSON - backported to 2.0.9 via fix: suggest setting flux Content-Type when query fails to parse as JSON #22548
• fix: for Windows, copy snapshot files being backed up - backported to 2.0.9 via fix: for Windows, copy snapshot files being backed up #22563
• fix(tsdb): sync series segment to disk after writing - unreleased change
• fix: allow empty request bodies to write API - backported to 2.0.9 via fix: allow empty request bodies to write API #22578
• fix: do not allow shard creation to create overlapping shards - NOT backported to 2.0.9 via fix: do not allow shard creation to create overlapping shards (#22604) #22609
• feat(query/stdlib): update push down window logic for location option - unreleased change
• feat: add --storage-write-timeout flag to set write request timeouts - unreleased change
• feat: show measurement database and retention policy wildcards - unreleased change
• feat: implement replication validation - partial feature implementation; no changelog entry needed
• feat: new recovery subcommand allows creating recovery user/token - unreleased change
• feat: return new operator token during backup overwrite - unreleased change
• fix: change session cookie name used by UI to avoid conflict with incompatible 2.0.x cookie - no user impact since the bug this fixed never existed in anything that was released
• feat(query/stdlib): update window planner rules for location changes to support fixed offsets - unreleased change
• fix(upgrade): don't drop shard-group durations when upgrading DBs - unreleased change
• feat(flux): enable writing to remote hosts via to() and experimental.to() - unreleased change
• feat: Add Bearer token auth - unreleased change
• feat: enable new dashboard autorefresh - unreleased change
• feat: list-bucket API supports pagination when filtering by org - unreleased change
• build(flux): update flux to v0.134.0 - unreleased change
• feat: update UI to OSS-2.1.0 - unreleased change

[williamhbaker]

Since these feature flags were all "on" by default, this change does not have a user impact.

[danxmoran]

Should we omit this since we ended up deciding not to add Geo support to OSS?

[williamhbaker]

Yeah that makes sense. Removed!

[danxmoran]

Could we elaborate on this? Or are we trying to be strict about CHANGELOG entry == commit message?

[williamhbaker]

I expanded on the description to be more clear. No need to match commit messages right now.

[danxmoran]

Side note: I think this might have been reverted in the 1.x line, @DStrand1 might know

[williamhbaker]

It was reverted on the 1.9 branch: #22605

Sounds like that was for the 1.9.5 release and it will be back in for later releases. The change is still exists in master-1.x.