pageserver: remove AncestorStopping error variants

[jcsp]

Problem

In all cases, AncestorStopping is equivalent to Cancelled.

This became more obvious in #7912 (comment) when updating these error types.

Summary of changes

• Remove AncestorStopping, always use Cancelled instead

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]

3156 tests run: 3017 passed, 0 failed, 139 skipped (full report)

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Flaky tests (1)

Postgres 14

• test_location_conf_churn[2]: release

Code coverage* (full report)

• functions: 31.4% (6493 of 20689 functions)
• lines: 48.4% (50220 of 103831 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
f4305e7 at 2024-05-31T14:52:28.674Z :recycle:}