Forbid creation of child timelines of archived timeline

[arpad-m]

We don't want to allow any new child timelines of archived timelines. If you want any new child timelines, you should first un-archive the timeline.

Part of #8088

[github-actions]

5038 tests run: 4874 passed, 0 failed, 164 skipped (full report)

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Flaky tests (10)

Postgres 17

• test_hot_standby_feedback: debug-x86-64
• test_pg_regress[None]: debug-x86-64
• test_readonly_node_gc: debug-x86-64
• test_scrubber_physical_gc_ancestors_split: debug-x86-64
• test_tenant_config: debug-x86-64
• test_subscriber_restart: debug-x86-64

Postgres 16

• test_hot_standby_feedback: release-x86-64
• test_slots_and_branching: release-x86-64

Postgres 15

• test_ondemand_wal_download_in_replication_slot_funcs: release-x86-64

Postgres 14

• test_ondemand_wal_download_in_replication_slot_funcs: release-x86-64

Code coverage* (full report)

• functions: 32.2% (7473 of 23243 functions)
• lines: 50.0% (60228 of 120515 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
adab332 at 2024-09-25T21:46:24.409Z :recycle:}

[skyzh]

LGTM :)