Dependabot: Single weekly PR for GH Actions, Single PR for terraform-plugin-* Go dependencies (#252)

* github: Group all Github action updates in a single Dependabot PR once a week

* dependabot: single PR for terraform-plugin-* dependencies

* dependabot: skip single PR for terraform-plugin-* dependencies for this repo as it's uncommon to happen at the same time or it's not even using those libraries

Author: ansgarm

.github/dependabot.yml

@@ -10,5 +10,12 @@ updates:
       interval: "daily"
   - package-ecosystem: "github-actions"
     directory: "/"
+    groups:
+      "github-actions":
+        patterns:
+          - "*" # Group all GitHub Actions dependencies together
     schedule:
-      interval: "daily"
+      interval: "weekly"
+      day: "monday"
+      time: "09:00"
+      timezone: "Etc/UTC"