Allow Gitaly to work without a PG connection

Gitaly didn't require a PG connection to connect to, though recently we
started to pass feature flags to Gitaly in different manners. That
change would now require a PG to be running. This is not ideal,
especially if only for feature flags.

This change rescues a bad connection error, and breaks the required
dependency between PG and Gitaly.

Closes https://gitlab.com/gitlab-org/gitlab/issues/34205

changelogs/unreleased/zj-pg-connection-ff-gitaly.yml

+---
+title: Remove required dependecy of Postgresql for Gitaly
+merge_request: 18659
+author:
+type: other

lib/feature/gitaly.rb

@@ -19,7 +19,7 @@ class Feature
 
         default_on = DEFAULT_ON_FLAGS.include?(feature_flag)
         Feature.enabled?("gitaly_#{feature_flag}", default_enabled: default_on)
-      rescue ActiveRecord::NoDatabaseError
+      rescue ActiveRecord::NoDatabaseError, PG::ConnectionBad
         false
       end