pageserver: reduce default compaction_upper_limit to 20

[erikgrinaker]

Problem

We've seen the previous default of 50 cause OOMs. Compacting many L0 layers at once now has limited benefit, since the cost is mostly linear anyway. This is already being reduced to 20 in production settings.

Summary of changes

Reduce DEFAULT_COMPACTION_UPPER_LIMIT to 20.

Once released, let's remove the config overrides.

[github-actions]

7546 tests run: 7174 passed, 0 failed, 372 skipped (full report)

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Flaky tests (4)

Postgres 17

• test_migration_to_cold_secondary: release-arm64-with-lfc
• test_subscriber_branching: debug-x86-64-without-lfc
• test_branched_from_many_empty_parents_size: debug-x86-64-without-lfc

Postgres 16

• test_change_pageserver: release-arm64-with-lfc

Code coverage* (full report)

• functions: 32.9% (8620 of 26193 functions)
• lines: 48.9% (72727 of 148858 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
895edd0 at 2025-02-19T12:45:13.907Z :recycle:}