Now that we've backported dbcheck to 8.0, we should drop the feature flag version here from 8.1 to 8.0 (note that the feature flag on 8.0 is enabled and already set to version 8.0). We should replace the tag requires_fcv_81 with requires_fcv_80 for any dbcheck check jstests, and ensure we are getting multiversion test coverage for dbcheck.