fix(pageserver): update remote latest_gc_cutoff after gc-compaction

[skyzh]

Problem

close #10208
part of #9114

Summary of changes

• Ensure remote latest_gc_cutoff is up-to-date before removing any files for gc-compaction.

[github-actions]

7095 tests run: 6796 passed, 0 failed, 299 skipped (full report)

---

Flaky tests (4)

Postgres 17

• test_nbtree_pagesplit_cycleid: debug-x86-64
• test_sharding_split_failures[failure19]: debug-x86-64
• test_tenant_creation_fails: debug-x86-64

Postgres 15

• test_physical_replication_config_mismatch_too_many_known_xids: release-arm64

Code coverage* (full report)

• functions: 31.2% (8396 of 26876 functions)
• lines: 47.9% (66656 of 139022 lines)

* collected from Rust tests only

---

_{The comment gets automatically updated with the latest test results
3979716 at 2024-12-19T18:36:18.518Z :recycle:}