WP Super cache: make sure the debug log viewer isn't deleted by garbage collection

[donnchawp]

The debug log viewer may be deleted by garbage collection if the file is older than the garbage collection TTL. The only way to fix it is by deleting the log from the Debug log page.
This PR recreates the viewer if missing, or touches it so it's modification time is updated when garbage collection is run, so it's always seen as a new file.

Fixes #32626

Proposed changes:

• In wp_cache_phase2_clean_expired() recreate or touch the log viewer php script so it's always new and won't be deleted by garbage collection.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

• Set your garbage collection timeout to 90 seconds, and GC timer to 60 seconds.
• Enable debugging on the Debug page and load the debug viewer.
• Wait a few minutes.
• Try to reload the debug viewer page. Make sure that garbage collection has run more than 90 seconds after debugging was enabled. Look for "Cache garbage collection." in the logs.

[github-actions]

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Super Cache plugin:

• Next scheduled release: August 6, 2024.
• Scheduled code freeze: July 29, 2024.

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[dilirity]