fix(pageserver): refresh_gc_info should always increase cutoff

[skyzh]

Problem

close https://github.com/neondatabase/cloud/issues/19671

Timeline -----------------------------
         ^ last GC happened LSN
              ^ original retention period setting = 24hr
> refresh-gc-info updates the gc_info
              ^ planned cutoff (gc_info)
         ^ customer set retention to 48hr, and it's still within the last GC LSN
         ^1   ^2 we have two choices: (1) update the planned cutoff to
                 move backwards, or (2) keep the current one

In this patch, we decided to keep the current cutoff instead of moving back the gc_info to avoid races. In the future, we could allow the planned gc cutoff to go back once cplane sends a retention_history tenant config update, but this requires a careful revisit of the code.

Summary of changes

Ensure that GC cutoffs never go back if retention settings get changed.

[github-actions]

5579 tests run: 5361 passed, 0 failed, 218 skipped (full report)

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Flaky tests (2)

Postgres 17

• test_location_conf_churn[3]: debug-x86-64
• test_pull_timeline_partial_segment_integrity: debug-x86-64

Code coverage* (full report)

• functions: 31.4% (7958 of 25346 functions)
• lines: 49.3% (63162 of 128163 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
d78e3a9 at 2024-11-22T22:34:41.817Z :recycle:}

[skyzh]

neither of the test failure is related to the patch itself... i suppose