ci: keep GitHub Actions up to date with GitHub's Dependabot (#42042)

* Keep GitHub Actions up to date with GitHub's Dependabot

Fixes software supply chain safety warnings like at the bottom right of
https://github.com/electron/electron/actions/runs/8619491231

* [Keeping your actions up to date with Dependabot](https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot)
* [Configuration options for the dependabot.yml file - package-ecosystem](https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#package-ecosystem)

* Apply suggestion from code review

* chore: fully remove groups

---------

Co-authored-by: David Sanders <dsanders11@ucsbalum.com>

.github/dependabot.yml

@@ -0,0 +1,9 @@
+# Keep GitHub Actions up to date with GitHub's Dependabot...
+# https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot
+# https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#package-ecosystem
+version: 2
+updates:
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: weekly