Setup Dependabot

Targeting 2.19.x, since we want the updates to bubble up. Since
Dependabot has had no effect on doctrine/dbal yet, I suppose that means
that "dependabot.yml" must be present on the default branch.

.github/dependabot.yml

@@ -0,0 +1,9 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    labels:
+      - "CI"
+    target-branch: "2.19.x"