Enable sec_secret_detection_ui_enable by default

What does this MR do?

Enables the sec_secret_detection_ui_enable by default and documents how Secret Detection can be enabled via an MR from the Security Configuration page.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required) #326716 (closed)
• 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

Related to #327028 (closed) and #326716 (closed)