There are two things we must add for featureFlagAggOutTimeseries

1. We do check the feature flag twice. This could cause a problem, but we are allowed to keep this check since we clean up if the FCV was downgraded in the middle of the test. We are asked to add a test confirming we clean up correctly. The test should be running $out with the current FCV, downgrade the FCV so the feature flag is off during the $out before we enter finalize(), then continue $out which should fail. We did have a test confirming the downgrade/upgrade behavior that was deleted when we branched 8.0, but it didn't confirm concurrent FCV downgrades (here is the test).
2. The flag does change what is written to disk because with the flag on we could write documents to a time-series collection. We hold a global IS lock in $out, defined in DocumentSourceWriter but we need to hold a global lock that conflicts with a S lock.