ci: configure dependabot to montly update GHAs

Signed-off-by: Diogo Teles Sant'Anna <diogoteles@google.com>

.github/dependabot.yml

+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "monthly"
+    groups:
+      github-actions:
+        patterns:
+          - "*"
+