Improve MR Experience for Secure Scans on Non-Ultimate Plans - Remove feature flag

Implementation plan

Feature flag: core_security_mr_widget

• Enable the feature flag in production for a couple of days
  • Go to #production or #chat-ops-test in Slack, and enter this command: /chatops run feature set core_security_mr_widget true
• Add documentation
• Assuming there are no issues, remove feature flag added in #249543 (closed) and associated code paths
• Delete the feature flag from production
  • Go to #production or #chat-ops-test in Slack, and enter this command: /chatops run feature delete core_security_mr_widget
  • Done