Updated feature flag YML files

https://gitlab.com/gitlab-org/gitlab/-/issues/251080

config/feature_flags/development/default_merge_ref_for_diffs.yml

 ---
 name: default_merge_ref_for_diffs
-introduced_by_url: 
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/34472
 rollout_issue_url:
 type: development
-group:
+group: group::source code
 default_enabled: false

config/feature_flags/development/merge_request_widget_graphql.yml

 ---
 name: merge_request_widget_graphql
-introduced_by_url: 
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/38311
 rollout_issue_url:
-group: 
+group: group::source code
 type: development
 default_enabled: false

config/feature_flags/development/view_diffs_file_by_file.yml

 ---
 name: view_diffs_file_by_file
-introduced_by_url: 
-rollout_issue_url: 
-group: 
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/35223
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/229848
+group: group::source code
 type: development
 default_enabled: true

config/feature_flags/development/widget_visibility_polling.yml

 ---
 name: widget_visibility_polling
-introduced_by_url: 
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/29318
 rollout_issue_url:
-group: 
+group: group::source code
 type: development
 default_enabled: true