release-25.2.6-rc: kvserver: invoke callback when SetMaxSize drops replicas

[wenyihu6]

Backport 1/1 commits from #153008.

/cc @cockroachdb/release

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Epic: none
Release note: none

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kvserver: invoke callback when dropping replicas in bq.SetMaxSize

Previously, when SetMaxSize shrank the queue, replicas were dropped from the
priority queue without invoking their callbacks. This commit ensures callbacks
are properly invoked when SetMaxSize drops replicas. Replicas removed via
removeFromReplicaSetLocked (such as when a replica is destroyed) still don’t
always have their callbacks invoked. While the natural place to invoke the
callback would be at removeFromReplicaSetLocked, invoking callbacks while
holding a lock risks blocking for too long. (We are doing this already for
addInternal though.) This PR focuses specifically on handling the SetMaxSize
case since this PR is intended to be backported. We can follow up with a more
complex but more principled approach on master if needed in the future.

Release justification: low risk fix guarded behind cluster setting

[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 #153180 does not comply with backport policy

Confidence: medium
Explanation: This PR is forwarded as a backport of a specific change from a master branch PR. The release justification is mentioned, but it is imprecisely identified as 'low risk fix guarded behind cluster setting'. However, no explicit mention or detail on the specific cluster setting or its default state is provided. Since it is only described as 'low risk', this doesn't categorically meet the critical bug fix norm according to the defined policy criteria for immediate backports. Additionally, without clear evidence of a cluster setting being used as a feature flag with a default non-active state, it's not possible to definitively ascertain compliance with the feature flag requirement. Changes are made to the production files 'pkg/kv/kvserver/queue.go' and 'pkg/kv/kvserver/queue_test.go'. The queue_test.go file, being a test, is exempt from feature flag requirements.
Recommendation: Reconsider the backport; Enhance justification or confirm the existence and default state of the cluster setting as a feature flag

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