Remove before_script from DAST vendored template

What does this MR do?

Remove before_script from DAST vendored template: moves dast_run under script section and remove. The before_script or after_script sections are prohibited to use in vendored templates as it conflicts with Auto DevOps.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/10471

Corresponding EE MR

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10248

Does this MR meet the acceptance criteria?

• [ ] Changelog entry added, if necessary
• [ ] Documentation created/updated via this MR
• [ ] Documentation reviewed by technical writer or follow-up review issue created
• [ ] Tests added for this feature/bug
• [ ] Tested in all supported browsers
• [ ] Conforms to the code review guidelines
• [ ] Conforms to the merge request performance guidelines
• Conforms to the style guides
• [ ] Conforms to the database guides
• ~~[ ] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.~~
• EE specific content should be in the top level /ee folder
• [ ] For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer