CephFS driver doesn't allow generic group snapshots

Description
===========
Manila's CephFS driver doesn't permit creating new share group snapshots. When attempted, we see an exception in the manila-share logs:

        ERROR manila.share.manager [None req-c146e845-edb4-4ff8-bab1-b2dd2a99240d 9733eebbc9d7436ab603733d4ef14389 a4d5fdde82d84d7bb2ac923db91098e1 - - - -] Share group snapshot 093ce239-ada9-4c6c-9a3c-4c04e274ef8a: create failed: manila.exception.ShareBackendException: Share backend error: Share group snapshot feature is no longer supported in mainline CephFS (existing group snapshots can still be listed and deleted).
        ERROR oslo_messaging.rpc.server [None req-c146e845-edb4-4ff8-bab1-b2dd2a99240d 9733eebbc9d7436ab603733d4ef14389 a4d5fdde82d84d7bb2ac923db91098e1 - - - -] Exception during message handling: manila.exception.ShareBackendException: Share backend error: Share group snapshot feature is no longer supported in mainline CephFS (existing group snapshots can still be listed and deleted).
        ERROR oslo_messaging.rpc.server Traceback (most recent call last):
        ERROR oslo_messaging.rpc.server File "/usr/lib/python3.9/site-packages/oslo_messaging/rpc/server.py", line 165, in _process_incoming
        ERROR oslo_messaging.rpc.server res = self.dispatcher.dispatch(message)
        ERROR oslo_messaging.rpc.server File "/usr/lib/python3.9/site-packages/oslo_messaging/rpc/dispatcher.py", line 309, in dispatch
        ERROR oslo_messaging.rpc.server return self._do_dispatch(endpoint, method, ctxt, args)
        ERROR oslo_messaging.rpc.server File "/usr/lib/python3.9/site-packages/oslo_messaging/rpc/dispatcher.py", line 229, in _do_dispatch
        ERROR oslo_messaging.rpc.server result = func(ctxt, **new_args)
        ERROR oslo_messaging.rpc.server File "/usr/lib/python3.9/site-packages/manila/utils.py", line 579, in wrapper
        ERROR oslo_messaging.rpc.server return func(self, *args, **kwargs)
        ERROR oslo_messaging.rpc.server File "/usr/lib/python3.9/site-packages/manila/share/manager.py", line 4852, in create_share_group_snapshot
        ERROR oslo_messaging.rpc.server LOG.error("Share group snapshot %s: create failed",
        ERROR oslo_messaging.rpc.server File "/usr/lib/python3.9/site-packages/oslo_utils/excutils.py", line 227, in __exit__
        ERROR oslo_messaging.rpc.server self.force_reraise()
        ERROR oslo_messaging.rpc.server File "/usr/lib/python3.9/site-packages/oslo_utils/excutils.py", line 200, in force_reraise
        ERROR oslo_messaging.rpc.server raise self.value
        ERROR oslo_messaging.rpc.server File "/usr/lib/python3.9/site-packages/manila/share/manager.py", line 4793, in create_share_group_snapshot
        ERROR oslo_messaging.rpc.server self.driver.create_share_group_snapshot(
        ERROR oslo_messaging.rpc.server File "/usr/lib/python3.9/site-packages/manila/share/drivers/cephfs/driver.py", line 681, in create_share_group_snapshot
        ERROR oslo_messaging.rpc.server raise exception.ShareBackendException(msg=msg)
        ERROR oslo_messaging.rpc.server manila.exception.ShareBackendException: Share backend error: Share group snapshot feature is no longer supported in mainline CephFS (existing group snapshots can still be listed and deleted).
        ERROR oslo_messaging.rpc.server

Steps to reproduce
==================

A chronological list of steps which will help reproduce the issue you hit:
* With the CephFS driver enabled (protocol doesn't matter), Create a share group
* Create one or more shares in the group
* Create a share group snapshot

Expected result
===============
Share group snapshot creation succeeds

Actual result
=============

Share group snapshot creation fails asynchronously with status set to "error".

Environment
===========
1. Exact version of OpenStack Manila you are running: Antelope; but the error seems to be existing since the Wallaby release

2. Which storage backend did you use? Ceph Quincy

3. Which networking type did you use N/A