Deprecate `PinSubVolumeGroup` in favour of `PinSubVolumeInGroup`

> Should we deprecate PinSubVolumeGroup since this call should be able to do everything PinSubVolumeGroup does (right)?

Good point.

> Even if yes, we don't have to do it right away.

I'll create an issue to be addressed once `PinSubVolumeInGroup` becomes stable.

_Originally posted by @anoopcs9 in https://github.com/ceph/go-ceph/issues/1305#issuecomment-5168921208_

Start by locating the PinSubVolumeGroup and PinSubVolumeInGroup API definitions and checking whether PinSubVolumeInGroup is stable. Review their usages and related tests or documentation; done means the project has agreed that the older call can be deprecated in favor of the stable replacement.

Written by the indexing model from the issue text.