Enable GH Actions to be automatically bumped

Turns out those get new major releases somewhat often, and we get deprecation warnings and brownouts and all that.
Automatic bumps should help us avoid that, though I've set a *very* large scan interval to shield us from churn.

.github/dependabot.yml

@@ -0,0 +1,14 @@
+# https://docs.github.com/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "quarterly" # We do not need *immediate* bumps, only frequent enough to avoid falling out of support range.
+    cooldown: # Avoid bumping deps immediately, to give time for security audits to be conducted
+      default-days: 15
+      include: [ * ]
+    groups:
+      actions:
+    labels: [ builds ] # Since those are CI-related updates...
+