fix(pageserver): use the larger one of upper limit and threshold

[skyzh]

Problem

Follow up of #10550 in case the upper limit is set larger than threshold. It does not make sense for someone to enforce the behavior like "if there are >= 50 L0s, only compact 10 of them".

Summary of changes

Use the maximum of compaction threshold and upper limit when selecting L0 files to compact.

[problame]

This hasn't caused problems, right? Because the default upper limit is larger than default threshold ?

[skyzh]

Yes, but we set the threshold for some tenants in prod to 100/300, which is larger than the default upper limit 50.

[github-actions]

7414 tests run: 7063 passed, 0 failed, 351 skipped (full report)

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Flaky tests (8)

Postgres 17

• test_pgdata_import_smoke[None-1024-RelBlockSize.MULTIPLE_RELATION_SEGMENTS]: debug-x86-64-without-lfc, release-arm64-with-lfc, release-arm64-without-lfc
• test_pgdata_import_smoke[8-1024-RelBlockSize.MULTIPLE_RELATION_SEGMENTS]: debug-x86-64-without-lfc, release-arm64-without-lfc, release-arm64-with-lfc
• test_timeline_archival_chaos: release-arm64-without-lfc

Postgres 14

• test_pageserver_gc_compaction_smoke[no_branches]: release-arm64-with-lfc

Code coverage* (full report)

• functions: 33.4% (8506 of 25499 functions)
• lines: 49.1% (71460 of 145505 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
c3da34e at 2025-01-30T00:15:26.880Z :recycle:}