Pin down the version of 'external-dns' to install

What does this MR do?

Pin down the external-dns version to be used for Review Apps to 2.6.1 as newer versions have an issue with AWS: https://github.com/kubernetes-sigs/external-dns/issues/1262

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

Availability and Testing

• The correct version is currently deployed: dns-gitlab-review-app 1 Mon Nov 25 18:35:50 2019 DEPLOYED external-dns-2.6.1 0.5.16 review-apps-ee

Closes #37269 (closed).