Specify the 'sync' policy for the external-dns deployments

What does this MR do?

This is to ensure that Review Apps DNS records are cleaned up when a Review App is itself tear-downed.

What are the relevant issue numbers?

Related to https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/5738.

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated
• [-] 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.
• Security reports checked/validated by reviewer