Replace custom filesystem backup sink with Go CDK one
We're using Go CDK library to access object storage as needed by backups. In addition to the built-in schemes, we're currently also supporting a custom filesystem sink here. Go CDK also offers a built-in object storage implementation that uses the filesystem as the backend.
It seems unnecessary to maintain our own filesystem sink when we could use the built-in filesystem backend from the library. We should drop the custom one and use the one provided by the library.