github: enable dependabot for github-actions

Turns out the dependabot tool can also indicate what github actions we
are using can be updated. This can be handy in avoiding the warnings
that github throws up all over an action that relies on a deprecated
feature. Use a monthly update just like go modules to avoid being
pestered all the time.

Signed-off-by: John Mulligan <jmulligan@redhat.com>

.github/dependabot.yml

@@ -10,3 +10,12 @@ updates:
       - "no-API"
     commit-message:
       prefix: "go-ceph"
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "monthly"
+    rebase-strategy: disabled
+    labels:
+      - "no-API"
+    commit-message:
+      prefix: "go-ceph"