doc/rados/configuration: Fix FileStore contradiction in journal-ref.rst

The page carried a deprecation warning on line 4 stating FileStore
"is no longer supported" but line 10 said "Filestore is preferred for
new deployments" -- a direct contradiction. Fix by:

- Expanding the warning to explicitly state FileStore must not be used
  for new deployments and to use BlueStore instead
- Updating the intro paragraph to reflect that BlueStore is the
  default and recommended back end, and that this page is for
  pre-existing Filestore OSDs pending migration only

Fixes: https://tracker.ceph.com/issues/77183

Signed-off-by: Emmanuel Ameh <eameh@contractor.linuxfoundation.org>