fix(pageserver): report timestamp is in the past if the key is missing

[skyzh]

Problem

If for some reasons we already garbage-collected the data under an LSN but the caller uses a past LSN for the find_time_cutoff function, now we will report a missing key error and GC will never proceed.

Note that missing key error can also happen if the key is really missing (i.e., during the past offload incidents)

Summary of changes

Make sure GC proceeds by bumping the LSN. However, data within the configured PiTR might not be accessible.

[github-actions]

7095 tests run: 6798 passed, 0 failed, 297 skipped (full report)

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Flaky tests (3)

Postgres 17

• test_timeline_archival_chaos: release-arm64

Postgres 16

• test_lr_with_slow_safekeeper: release-x86-64

Postgres 14

• test_physical_replication_config_mismatch_max_locks_per_transaction: release-x86-64

Code coverage* (full report)

• functions: 31.2% (8398 of 26876 functions)
• lines: 47.9% (66647 of 139020 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
1ae01d8 at 2024-12-19T19:02:21.319Z :recycle:}