Resolve "Remove feature flag :use_legacy_pipeline_triggers"

What does this MR do?

This MR is a combination of two MRs: !20715 (closed) & !21169 (closed) .

This MR removes feature flag use_legacy_pipeline_triggers. Any existing token without user (legacy token) will fail after this MR.

It will add a post-migration to delete all legacy triggers which has no owner_id. It also adds NOT NULL constraint to owner_id column of ci_triggers.

ref: #29772 (closed)

Screenshots

After migration, 6972 rows will be deleted from ci_triggers table.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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

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