Remove ubuntu images from dependabot

We could pin the major and minor versions, but may not work dependabot/dependabot-core#6115.
Instead, because there is no patch version, dependabot should actually
never make a PR for these Dockerfile. Thus we can ignore them entirely.

.github/dependabot.yml

@@ -16,10 +16,14 @@ updates:
     directory: /packaging/docker/ubuntu-18.04
     schedule:
       interval: "monthly"
+    ignore:
+      - dependency-name: ubuntu
   - package-ecosystem: "docker"
     directory: /packaging/docker/ubuntu-20.04
     schedule:
       interval: "monthly"
+    ignore:
+      - dependency-name: ubuntu
   - package-ecosystem: "docker"
     directory: /.buildkite
     schedule: