Boost: Fix removing WP_CACHE when advanced cache wasn't deleted

[dilirity]

Boost should only clean up after itself it it's the last thing that
enabled caching. If it cleans up, it might accidentally break caching
for other plugins.

Fixes #38032

Proposed changes:

• Prevent Boost from breaking caching for other plugins;
• Add comment cleanup for WP_CACHE constant in wp-config.php (didn't break anything, it just seems odd having a boost related comment when boost might not even be present on a site).

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

n/a

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

no

Testing instructions:

Test that deactivating Boost doesn't remove WP_CACHE from wp-config.php when it wasn't the last time that had cache enabled

1. Setup WP Super Cache and enable easy caching;
2. Setup Boost plugin and enable caching;
3. Check wp-config.php to make sure the WP_CACHE declaration got moved to the top of the file and there's a comment behind it (// Boost Cache Plugin);
4. Activate WP Super Cache (as it got deactivated in step 2);
5. Keep Boost plugin active, but disable caching;
6. Enable WP Super Cache caching;
7. Deactivate Boost plugin but don't visit WP Super Cache's settings (it will muddy the test, as it automatically adds constants to wp-config.php);
8. Check wp-config.php to make sure that WP_CACHE did not get removed (the comment from step 3 should be removed).

[github-actions]

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Boost plugin:

• Next scheduled release: July 2, 2024.
• Scheduled code freeze: June 24, 2024.

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[donnchawp]

Works as expected! Thanks!

[dilirity]

No cherry pick necessary. We ended up cutting a new release from trunk.