pageserver: refactor immediate_gc into TenantManager

[jcsp]

Problem

Legacy functions that were called as mgr:: and relied on the static TENANTS, see #5796

Summary of changes

• Move the last stray function (immediate_gc) into TenantManager

Closes: #5796

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

5013 tests run: 4855 passed, 0 failed, 158 skipped (full report)

---

Flaky tests (4)

Postgres 17

• test_multi_attach: debug-x86-64

Postgres 16

• test_subscriber_restart: release-x86-64

Postgres 15

• test_ondemand_wal_download_in_replication_slot_funcs: release-x86-64

Postgres 14

• test_hot_standby_gc[False]: release-arm64

Code coverage* (full report)

• functions: 32.0% (7492 of 23433 functions)
• lines: 49.9% (60494 of 121116 lines)

* collected from Rust tests only

---

_{The comment gets automatically updated with the latest test results
b926e62 at 2024-09-27T17:48:08.267Z :recycle:}