Keep action versions up to date with Dependabot

This enables grouped Dependabot version updates, for GitHub Actions
only, to keep actions up to date. Currently some actions are used
at very old versions; for example, this uses `actions/checkout@v1`,
but the current version is `v4`. Such old versions are no longer
maintained and usually run on an old unsupported Node.js version.

Author: EliahKagan

.github/dependabot.yml

@@ -0,0 +1,9 @@
+version: 2
+updates:
+- package-ecosystem: github-actions
+  directory: '/'
+  schedule:
+    interval: weekly
+  groups:
+    github-actions:
+      patterns: ['*']