Discourage feature flags on security MRs

What does this MR do?

Adds a new check on danger/feature_flags/Dangerfile to prevent the usage of feature flags on security merge requests.

As part of gitlab-com/gl-infra/delivery#1739 (closed)

Depends on gitlab-org/release/docs!367 (merged)

Does this MR meet the acceptance criteria?

Conformity

• [-] I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• [-] I have added/updated documentation, or it's not needed. (Is documentation required?)
• [-] I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• [-] I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)

Testing

Status	MR
✅ Danger succeeded	MR on Canonical with feature flag
❌ Danger failed	MR on Security with a feature flag