gh: change github-actions dependabot updates to monthly cadence

to avoid PRs getting generated "all the time", move to a monthly
cadence for dependabot triggered github-actions updates.

Signed-off-by: Mikko Ylinen <mikko.ylinen@intel.com>

.github/dependabot.yml

@@ -4,6 +4,5 @@ updates:
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule:
-      # Check for updates to GitHub Actions every week on Saturday
-      interval: "weekly"
-      day: "saturday"
+      # Check for updates to GitHub Actions monthly, on the first day of the month
+      interval: "monthly"