pageserver: don't periodically flush layers for stale attachments

[erikgrinaker]

Problem

Tenants in attachment state Stale can't upload layers, and don't run compaction, but still do periodic L0 layer flushes in the tenant housekeeping loop. If the tenant remains stuck in stale mode, this causes a large buildup of L0 layers, causing logging, metrics increases, and possibly alerts.

Resolves #11245.

Summary of changes

Don't perform periodic layer flushes in stale attachment state.

[github-actions]

7964 tests run: 7580 passed, 0 failed, 384 skipped (full report)

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Code coverage* (full report)

• functions: 32.4% (8731 of 26955 functions)
• lines: 48.4% (74851 of 154549 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
4904091 at 2025-03-19T21:32:59.507Z :recycle:}