Pin the version of deployed external-dns to `2.y.z`, using SemVer
matching. (`^2.2.1`)

Update the properties for the service account keys per
*Upgrading to 2.0.0* of the `stable/external-dns` README.

Add `aws.batchChangeSize=400` to combat messages about RDATA being
too large.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/64356