pageserver: downgrade stale generation messages to INFO

[jcsp]

Problem

When generations were new, these messages were an important way of noticing if something unexpected was going on. We found some real issues when investigating tests that unexpectedly tripped them.

At time has gone on, this code is now pretty battle-tested, and as we do more live migrations etc, it's fairly normal to see the occasional message from a node with a stale generation.

At this point the cognitive load on developers to selectively allow-list these logs outweighs the benefit of having them at warn severity.

Closes: #8080

Summary of changes

• Downgrade "Dropped remote consistent LSN updates" and "Dropping stale deletions" messages to INFO
• Remove all the allow-list entries for these logs.

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

3006 tests run: 2891 passed, 0 failed, 115 skipped (full report)

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Code coverage* (full report)

• functions: 32.7% (6933 of 21207 functions)
• lines: 50.1% (54437 of 108678 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
4bb2e5f at 2024-07-03T23:28:56.953Z :recycle:}