[5.x] Use StaticCache facade in Cache Manager utility

[duncanmcclean]

This pull request changes how the Cache Manager utility clears the static cache, to bring it inline with the static:clear command.

Currently, the static:cache command calls the StaticCacheManager::flush() method. However, the "Clear" button on the cache manager utlity calls the cacher's flush method (eg. FileCacher::flush() or ApplicationCacher::flush()).

The flush method on the StaticCacheManager does end up calling the cacher's flush method but it also deals with clearing other static caching related stuff from the cache (like nocache URLs).

This isn't causing any issues right now, however, as part of #10403, I'm adding a StaticCacheCleared event which I'd like to be able to dispatch in one place (the StaticCacheManager, rather than in two places).