Blog: Clarify tier availability for PAT revocation

Why is this change being made?

Our blog from early January had an incorrect implication that was missed during writing and review. The feature in fact is limited to Ultimate projects but the blog indicated that all tiers were protected. The limitation is due to technical reasons (gitlab-org/gitlab#391763) and not due to any intention to limit the reach of the feature.

We have since corrected documentation and release posts (15.6 (beta), 15.9 (GA)) but should also fix it in the blog for any future readers.

Author Checklist

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