Pin to the major version of DAST

What does this MR do?

Converts the DAST vendored template to pin to the Major version of DAST, instead of the previous CI_SERVER_VERSION_MAJOR-$CI_SERVER_VERSION_MINOR-stable (e.g. 12-5-stable).

This helps decouple the DAST release process from the GitLab release process. It also gives the user some level of control about the kind of automatic DAST updates they want (feature, patch). It resolves an issue that existed as the previous DAST release strategy updated all x-x-stable releases, meaning a change in format of the DAST entrypoint would break old versions of GitLab trying to run DAST.

This in part resolves issue #33364 (closed).

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