Add Dependabot for Github actions

Dependabot will submit pull requests
when Github actions get new versions
released. After merging this, Dependabot
would have opened a pull request that
is identical to TerriaJS#7445 automatically.

Link to Dependabot documentation:

https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot

.github/dependabot.yml

@@ -0,0 +1,7 @@
+version: 2
+updates:
+  - package-ecosystem: github-actions
+    directory: "/"
+    schedule:
+      interval: "daily"
+    target-branch: "main"