exp/ingest/io: Remove support for TransactionMeta.V=0

[bartekn]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• This PR's title starts with name of package that is most changed in the PR, ex.
  services/friendbot, or all or doc if the changes are broad or impact many
  packages.

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or fixes.
• I've updated any docs (developer docs, .md
  files, etc... affected by this change). Take a look in the docs folder for a given service,
  like this one.

Release planning

• I've updated the relevant CHANGELOG (here for Horizon) if
  needed with deprecations, added features, breaking changes, and DB schema changes.
• I've decided if this PR requires a new major/minor version according to
  semver, or if it's mainly a patch change. The PR is targeted at the next
  release branch if it's not a patch change.

What

This commit removes support for ingesting TransactionMeta.V=0 in io.LedgerTransaction.GetChanges.

Close #2097.

Why

The code generating V0 meta was changed in stellar-core but without upgrading the V param. Some V0 meta has only one LEDGER_ENTRY_STATE emitted only when a ledger entry is first modified in a ledger but others add it before every LEDGER_ENTRY_UPDATE and LEDGER_ENTRY_REMOVED. It looks like the code to parse meta to support both cases should be possible but since transaction meta in protocol version <10 can be incomplete (see #1902) users still need to switch to V2 to be able to parse it.

See:

• Emit ledger entry in meta before modifying it stellar-core#944
• always emit previous state in meta stellar-core#1441

Known limitations

TransactionMeta generated by the older versions of stellar-core will not be supported but the current code is in LedgerTransaction.GetChanges is broken anyway.