Enable CSIVolumeGroupSnapshot feature gate by default

[xing-yang]

Is your feature request related to a problem?/Why is this needed

Describe the solution you'd like in detail

We moved volume group snapshot to beta while disabled the CSIVolumeGroupSnapshot feature gate by default:
#1194

We can enable it by default in the next release.

Describe alternatives you've considered

Additional context

[pwschuurman]

We can enable it by default in the next release.

Just to confirm, would this be the v9.0.0 release?

[xing-yang]

We are moving this feature to beta while keeping the feature gate disabled by default in the coming release which should be v9.0.0.
So maybe we can make it enabled by default in 9.1.0. (This is TBD)