Use `wp_filesize` instead of `filesize` if available

[felixarntz]

WordPress 6.0 introduced the wp_filesize() function which should be used in favor of PHP's filesize() going forward. We should use it in this plugin where available.

Since we currently support WordPress 5.8 and 5.9 as well, it's probably not worth bumping up the requirement just for that. I suggest we instead create a backward compatibility layer. Maybe the easiest approach would be to implement a "polyfill", where we re-implement wp_filesize() in the main plugin file if the function does not exist (yet).

We can then update all module code that calls filesize() to instead unconditionally call wp_filesize().