Merge branch 'ps-add-sourcegraph-feature-flag-yml' into 'master'

Add sourcegraph feature flag yml

See merge request gitlab-org/gitlab!49442

config/feature_flags/development/sourcegraph.yml

+---
+name: sourcegraph
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/16556
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/292199
+milestone: '12.5'
+type: development
+group: group::editor
+default_enabled: false