fix: filteredLocales in the client config are stale (#14326)

Fixes https://github.com/payloadcms/payload/issues/14299

Using `filterAvailableLocales` means they could change when the req
changes. When we get the client config from the cache, we need to
re-apply `filterAvailableLocales` so the config has the correct values.

This was manually being done in the Root layout, but every other place
that called getClientConfig was not i.e. the Root view. So this PR moves
that logic from the Root layout into the getClientConfig function.