release-24.1: changefeedccl: fix unwatched column families memory monitoring bug

[andyyang890]

Backport 1/1 commits from #154802.

/cc @cockroachdb/release

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This patch fixes a bug where the memory monitor wouldn't reclaim the
memory allocated to events corresponding to unwatched column families
for a changefeed that targets only a subset of a table's families.

Fixes #154776

Release note (bug fix): A bug where a changefeed targeting only a subset
of a table's column families could become stuck has been fixed.

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Release justification: fix for high-severity bug

[blathers-crl]

Thanks for opening a backport.

Before merging, please confirm that it falls into one of the following categories (select one):

• Non-production code changes. Includes test-only changes, build system changes, etc.
• Fixes for serious issues. Defined in the policy as correctness, stability, or security issues, data corruption/loss, significant performance regressions, breaking working and widely used functionality, or an inability to detect and debug production issues.
• Other approved changes. These changes must be gated behind a disabled-by-default feature flag unless there is a strong justification not to.

Add a brief release justification to the PR description explaining your selection.

Also, confirm that the change does not break backward compatibility and complies with all aspects of the backport policy.

All backports must be reviewed by the TL and EM for the owning area.

[cockroach-teamcity]

This change is 

[blathers-crl]

✅ PR #155025 is compliant with backport policy

Confidence: high
Critical bug criteria met: [Bugs that can cause the DB to return incorrect results or result in suboptimal performance]
Backward compatible: true
Explanation: The pull request is compliant with the backport policy for the following reasons:

1. Release Justification: The PR body contains a 'Release justification' line stating it is a 'fix for high-severity bug,' which aligns with the backport policy for critical bug fixes.
2. Critical Bug: The PR addresses a bug where a changefeed targeting only a subset of a table's column families could become stuck. This fits the critical bug criteria as it fixes a bug that can cause suboptimal performance and potentially affects the stability of the system.
3. File Changes: The changes include modifications to production code that address the specific bug as described, without introducing backward compatibility issues or removing version gates.
4. Testing Evidence: The addition of tests in 'changefeed_memory_test.go' supports the claim and nature of the bug fix, demonstrating dedication to validating the fix adequately.

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