Clarify case normalization in db spec

[gregkalapos]

Fixes #886

Changes

Adding note about case normalization to following attributes:

• db.collection.name
• db.namespace
• db.operation.name

The added note suggested in #886

It is RECOMMENDED to capture the value as provided by the application without attempting to do any case normalization.

Merge requirement checklist

• CONTRIBUTING.md](https://github.com/open-telemetry/semantic-conventions/blob/main/CONTRIBUTING.md) guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• schema-next.yaml updated with changes to existing conventions.

[trask]

I think probably needs a change log

[gregkalapos]

CI fails with:

34s
Run make markdown-link-check
  make markdown-link-check
  shell: /usr/bin/bash -e {0}
fatal: No names found, cannot describe anything.
semantic-conventions@ /home/runner/work/semantic-conventions/semantic-conventions
└── markdown-link-check@3.11.2


  ERROR: 1 dead links found in ./docs/attributes-registry/db.md !
  [✖] https://docs.datastax.com/en/cassandra-oss/3.0/cassandra/dml/dmlConfigConsistency.html → Status: 404
make: *** [Makefile:47: markdown-link-check] Error 1
Error: Process completed with exit code 2.

That link works from my browser - and the PR doesn't really change anything around it. Strange. Trying to figure this out.