Dependabot: update config

This commit makes two changes to the Dependabot config:
1. It introduces a "cooldown" period for updates to a new major release of action runners.
    What this means, is that for updates to a new major, the Dependabot will be delayed by 10 days, which should give projects the chance to fix any "teething problems".
2. It introduces a "group".
    By default Dependabot raises individual PRs for each update. Now, it will group updates to new minor or patch release for all action runners into a single PR.
    Updates to new major releases of action runners will still be raised as individual PRs.

Refs:
* https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/optimizing-pr-creation-version-updates
* https://docs.github.com/en/code-security/dependabot/working-with-dependabot/dependabot-options-reference

Author: jrfnl

.github/dependabot.yml

@@ -27,3 +27,11 @@ updates:
       prefix: "GH Actions:"
     labels:
       - "Type: chores/QA"
+    cooldown:
+      semver-major-days: 10
+    groups:
+      action-runners:
+        applies-to: version-updates
+        update-types:
+          - "minor"
+          - "patch"