Remove beta note from projected workspaces and csi as they are stable

[BastiaanN]

Changes

In the documentation csi and projected volume workspaces still have a note saying it is a beta feature.
This is incorrect as both features were promoted to stable a while ago.

See the following PR: #6954

Submitter Checklist

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• Has Docs if any changes are user facing, including updates to minimum requirements e.g. Kubernetes version bumps
• Has Tests included if any functionality added or changed
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Release Notes

NONE

[BastiaanN]

/kind documentation

[BastiaanN]

/assign @abayer

[waveywaves]

/lgtm

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[afrittoli]

/lgtm