Move the long running test to master only

What does this MR do?

Updates the DAST pipeline to run the memory end-to-end test only on the master branch.

This is deemed appropriate given that the test takes around 20 minutes to run and has a low likelihood of failure.

What are the relevant issue numbers?

gitlab-org/gitlab#231326 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Job definition example
  • Vendored CI Templates (also in CE)
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer