FSGroup CSI delegation KEP: update metadata, update feature gate name to the latest version

[verult]

• One-line PR description: update KEP metadata for beta; update feature gate name to the latest version

• Issue link: Allow Kubernetes to supply pod's fsgroup to CSI driver on mount #2317

• Other comments:

/assign @gnufied @salaxander
/sig storage

[gnufied]

/lgtm

[xing-yang]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: verult, xing-yang
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[msau42]

Hm how did this merge without prr approval?

[verult]

The corresponding PRR PR containing everything here also merged, that might've been why: #2941

[xing-yang]

Yes, #2941 includes the commit from this PR.