.github/dependabot.yml: add monthly Actions checks

We schedule monthly checks by the GitHub Dependabot service of
our CI workflow to ensure we are using the latest available versions
of all GitHub Actions steps.

h/t jlosito for the suggestion in git-lfs/git-lfs#5228

Author: chrisd8088

.github/dependabot.yml

@@ -0,0 +1,7 @@
+---
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "monthly"