Resolve "DAST for the master branch"

Docs are in a separate MR: !18232 (merged)

What does this MR do?

This MR enables DAST on default branches.

Disabling DAST on the default branch

DAST on the default branch can be disabled by setting a DAST_DISABLED_FOR_DEFAULT_BRANCH environment variable to "true".

The review app

DAST will use its own review app on the default branch, and the existing review app on any other branch.

Screenshots

Before

This is what the pipeline looked like on a default branch before these changes

After

This is what the pipeline looks like on a default branch with DAST.

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

See #5503 (closed)