Add merge request URL to yaml files

Add MR URLs to the feature flag and changelog
files.

config/feature_flags/development/usage_data_group_code_coverage_visit_total.yml

 ---
 name: usage_data_group_code_coverage_visit_total
-introduced_by_url:
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/50790
 rollout_issue_url:
 milestone: '13.8'
 type: development

ee/changelogs/unreleased/224617-usage-data-generic-metrics-report.yml

 ---
 title: Capture generic metrics report views via usage ping
-merge_request:
+merge_request: 50790
 author:
 type: added