XWikiCacheStoreInterface#flushCache should always be called after a migration

The migrations we're performing usually have an impact to the documents, and it's very easy to forget to flush the cache when performing a migration, which might then be overridden by another migration if it was done directly in DB and the other migration use the cached doc to perform changes.
This is what happened in XWIKI-25199, so to avoid having that issue in the future I think we should systematically flush the cache after a migration.