Add missing x509_forced_cert_loading ff definition file [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Adds missing x509_forced_cert_loading feature flag definition file.

While working on !56746 (merged) I've found a couple of missing feature flag definition files.

There is a danger job, that requires a group in the definition file to be matched the label group in the MR. That's why there is a separate MR for adding that definition file.

Screenshots (strongly suggested)

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 a feature flag definition file was added.
• 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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team