Automatically update GitHub Actions with Dependabot (#111)

GitHub Actions runners periodically update to a new version of Node,
causing a cascade of major version bumps in various actions (because
this is a breaking change on self-hosted runners) that are annoying to
manually apply.  Let Dependabot do it, in a batched PR once a month.

Author: bgilbert

.github/dependabot.yml

@@ -0,0 +1,12 @@
+version: 2
+updates:
+  - package-ecosystem: github-actions
+    directory: /
+    commit-message:
+      prefix: workflows
+    groups:
+      actions:
+        patterns: ["*"]
+    labels: ["dependencies"]
+    schedule:
+      interval: monthly