Configure Dependabot to keep actions up to date

Author: djc

.github/dependabot.yml

@@ -6,3 +6,8 @@ updates:
     interval: weekly
     time: "13:00"
   open-pull-requests-limit: 99
+- package-ecosystem: github-actions
+  directory: "/"
+  schedule:
+    interval: weekly
+    time: "05:23"