Enable core_security_mr_widget feature flag by default

What does this MR do?

Enables the core_security_mr_widget feature flag by default, added by !44639 (merged).

Addresses #249550 (closed), part of &4393 (closed).

Screenshots

n/a, though see !44639 (merged) which implemented the feature.

Draft

This is marked as Draft since it's based on and blocked by !44639 (merged). ✅

Once !44639 (merged) has merged:

1. This MR's branch needs to be rebased onto master ✅
2. This MR needs to be edited to target the master branch (it currently targets the 249543-add-core-mr-security-widget branch) ✅

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done