featureflag: Default enable `FetchSourceBranchQuarantined`
requested to merge 4698-featureflag-enable-use-of-quarantined-repository-in-fetchsourcebranch-2 into master
Currently the flag is default disabled. Given the risk of
FetchSourceBranchQuarantined, the flag will be kept for another
release (15.9) and deprecated after (15.10). Let's default enable it so
that even customers would also hit it.
Part of #4698 (closed)