Replace Approval gates term with Status checks

What does this MR do?

It became apparent that the term "Approval gates" was confusing and not truly representative of the feature being built, so 🤝 groupcompliance groupsource code groupcode review has made the decision to rename this to "Status checks" and revise some of the other associated text.

This MR updates the relevant code and UI text.

Setup & Testing

1. Enable the development feature flag: echo "Feature.enable(:ff_compliance_approval_gates)" | rails c
2. View Merge request approvals at [GDK_HOST]/gitlab-org/gitlab-test/edit

Screenshots (strongly suggested)

View	Before	After
Add form
Edit form
Tooltip
Delete modal

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 it is behind the :ff_compliance_approval_gates feature flag.
• [-] 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

Related to #327627 (closed)