Update milestone for prevent account deletion to 16.1

What does this MR do?

!120106 (merged) added documentation on how Administrators can prevent users from deleting their account with a new user restrictions configuration setting.

!120749 (merged) updated the feature documentation to reflect that it is only available on GitLab Premium and GitLab Ultimate, as per this comment.

This MR updates the documentation to state that it is available from %16.1 rather than %16.0 as per this comment, and that it is enabled by default as per this comment.

Related issues

#26053 (closed)

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