CI/review_apps: Make use of external-dns and ephemeral IPs

Don't rely on static IPs for Ingress/LoadBalancers. To do this, we
implement annotations for `external-dns` to be able to populate
endpoints into our `$REVIEW_APPS_DOMAIN`.

This requires the installation of `external-dns` into the cluster
used by CI, which is not a part of this MR.

See #7683 comments for details of installation.

scripts/review_apps/review-apps.sh

@@ -128,10 +128,10 @@ HELM_CMD=$(cat << EOF
     --set releaseOverride="$CI_ENVIRONMENT_SLUG" \
     --set global.hosts.hostSuffix="$HOST_SUFFIX" \
     --set global.hosts.domain="$REVIEW_APPS_DOMAIN" \
-    --set global.hosts.externalIP="$REVIEW_APPS_DOMAIN_IP" \
     --set certmanager.install=false \
     --set global.ingress.configureCertmanager=false \
     --set global.ingress.tls.secretName=tls-cert \
+    --set global.ingress.annotations."external-dns\.alpha\.kubernetes\.io/ttl"="10"
     --set gitlab.unicorn.resources.requests.cpu=200m \
     --set gitlab.sidekiq.resources.requests.cpu=100m \
     --set gitlab.gitlab-shell.resources.requests.cpu=100m \