pageserver: switch on new-style local layer paths

[jcsp]

We recently added support for local layer paths that contain a generation number:

• pageserver: include generation number in local layer paths #7609
• pageserver: local layer path followups #7640

Now that we've cut a release that includes those changes, we can proceed to enable writing the new format without breaking forward compatibility.

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]

3060 tests run: 2933 passed, 0 failed, 127 skipped (full report)

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Code coverage* (full report)

• functions: 31.4% (6332 of 20168 functions)
• lines: 47.3% (47822 of 101105 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
e36a30d at 2024-05-13T17:21:49.068Z :recycle:}