chore(pageserver): use kebab case for aux file flag

[skyzh]

Problem

part of #7462

Summary of changes

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

[arpad-m]

we should maybe do this for compaction as well, currently we use "Tiered" and "Legacy".

[github-actions]

3132 tests run: 3005 passed, 0 failed, 127 skipped (full report)

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Flaky tests (2)

Postgres 15

• test_crafted_wal_end[last_wal_record_xlog_switch_ends_on_page_boundary]: debug

Postgres 14

• test_timeline_ancestor_errors: release

Code coverage* (full report)

• functions: 31.4% (6416 of 20458 functions)
• lines: 48.1% (49333 of 102645 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
1e7b758 at 2024-05-22T17:14:05.542Z :recycle:}