featureflag: Enable Go implementation of UserSquash

Feature flag 'gitaly_go_user_squash' is enabled by default.

Part of: #3258 (closed)

The feature flag was enabled for quite some time and there are no failures or issues reported about it, so we could enable it by default now and monitor further.

https://dashboards.gitlab.net/d/000000199/gitaly-feature-status?orgId=1&refresh=10s&from=now-1h&to=now&var-environment=gprd&var-method=UserSquash&var-prometheus=prometheus-01-inf-gprd

https://log.gprd.gitlab.net/app/kibana#/discover?_g=h@f7fb346&_a=h@49a70be