CMP0128: Enable/disable extensions if standard same as default

This was intended to be part of the initial MR (!6177 (merged)), but accidentally went missing when debugging nightly failures on less common systems. Noticed during !6711 (merged) review as the comment about this behaviour didn't match the code.

Documentation for CMP0128 is updated to note this case.
Cc @craig.scott for documentation review.

Fixes: #22224 (closed)
Backport: release
Topic-rename: cmp0128-fixup