CMP0128: Add flag in OLD mode even when standard matches the default

Commit 4a0485be (cmStandardLevelResolver: Avoid unnecessary flags, fix unset
level logic, 2021-04-29) unintentionally changed the behavior by modifying the
code to match a pre-existing comment. The resulting behavior change however
matches the intentions of CMP0128, so we simply need to guard it.

Fixes #23122.