WP Super Cache: fix clearing the cache when scheduled posts are published

[donnchawp]

A recent change added an "is backend" check to the code very early on in the "boot up" process of the request. One "is backend" URL is the website CRON function. This stopped the plugin hooking various functions that did maintenance of the cache. One of those was the transition action that fires when a post goes from scheduled to publish.
That stopped scheduled posts from clearing the cache as the caching system was disabled when CRON jobs ran.

Fixes #38257

Proposed changes:

• Remove the is_backend check from wp_cache_postload()

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

p1720508923710769-slack-C016BBAFHHS

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

No

Testing instructions:

• This is possibly easier to debug if you disable the WP cron and call it from the system cron as it's more predictable, especially on a quiet website.
• Enable the debug log.
• Schedule a post for 2 minutes into the future.
• Keep an eye on the debug log.
• When the post is "published", the debug log will show something like this when the post is published: wp_cache_post_edit: Clearing cache for post X on clean_post_cache
• Reload the posts page and the new post should be there.

[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]