docs: update PersistentReservation to GA, replace gate with config

[akalenyu]

What this PR does / why we need it:
Update persistent reservation docs to reflect the GA promotion.
Replace the developerConfiguration feature gate with the new
persistentReservationConfiguration API and fix typos.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

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• Upgrade: Impact of this change on upgrade flows was considered and addressed if required
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• Documentation: A user-guide update was considered and is present (link) or not required. You want a user-guide update if it's a user facing feature / API change.
• Community: Announcement to kubevirt-dev was considered

Release note:

NONE

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