Add can-load.php files to load Site Health checks for persistent cache and full page cache conditionally #540

Since the two modules for the Site Health checks for persistent cache and full page cache got merged into core, we need to add logic so that these two modules are only loaded going forward when the relevant core functionality is not already present. For that we should use the can-load.php approach which was established in #293.

Relevant WordPress core commits:

• https://core.trac.wordpress.org/changeset/53955
• https://core.trac.wordpress.org/changeset/54043

Since the class that has all the changes is only loaded within WP Admin, we may not be able to check for actual functions or methods reliably, as our plugin initialization would run too early. So potentially we will simply need to check that the WordPress version is lower than 6.1.