Block external pull mirrors for forks

Prevent forked projects from having pull mirrors configured
for outside of the fork network. This still allows mirroring
inside the fork network, and the root of the network can have
an external mirror.

Changelog: changed
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/60735

config/feature_flags/development/block_external_fork_network_mirrors.yml

@@ -5,4 +5,4 @@ rollout_issue_url:
 milestone: '14.0'
 type: development
 group: group::source code
-default_enabled: false
+default_enabled: true