Deprecate `PinSubVolumeGroup` in favour of `PinSubVolumeInGroup`

[anoopcs9]

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 #1305 (comment)

[anoopcs9]

PinSubVolumeInGroup can do everything PinSubVolumeGroup does since it operates on subvolumes within a group, which covers the same functionality. Once PinSubVolumeInGroup is considered stable, we should deprecate PinSubVolumeGroup and guide users to the newer API.