Clear update caches when site language is changed.

The update screen has a re-install button which could show the wrong language locale after the site language was changed. That's because the update data is cached. 
Introduce `_wp_clear_update_cache()` as a helper function to clean up the update cache.

props SergeyBiryukov, ocean90.
fixes #30369.

git-svn-id: https://develop.svn.wordpress.org/trunk@30554 602fd350-edb4-49c9-b593-d223f7449a82