Remove support for legacy pipeline triggers

What does this MR do?

The scope of this MR is to move support for legacy triggers behind a feature flag and remove it entirely in 2-3 coming releases.

Fixes: https://gitlab.com/gitlab-org/gitlab-ce/issues/30231

Feature Flag enabled

Keep the legacy behavior for enterprises that want to buy more time in migrating legacy triggers. The feature flag will enable them to admin and use legacy triggers and make them fully visible.

Feature Flag disabled

Drop support for legacy triggers entirely. Users won't be able to use or admin legacy triggers and they'll show as invalid until deleted. This will be the behavior deployed on Gitlab.com.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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