See HELP-53040 for motivation, as well as the docs.

There was an oversight in PM-1856 where the logic to decide whether or not to validate . and $ fields is only dependent on the feature flag, but not the FCV version (see the scope of PM-1856).

The fix for this should involve updating all checks to use `isEnabled()` instead of `isEnabledAndIgnoreFCV()`, and adding a test to verify that the FCV behavior is correct when upgrading/downgrading.