Create a Dependabot group for all GitHub actions

Make Dependabot update all dependencies in a single pull request.

Suggested by Pedro Nacht.

.github/dependabot.yml

diff --git a/.github/dependabot.yml b/.github/dependabot.yml
index 4eb1d53..322a2b6 100644
--- a/.github/dependabot.yml
+++ b/.github/dependabot.yml
@@ -9,3 +9,7 @@
     directory: "/"
     schedule:
       interval: "monthly"
+    groups:
+      github-actions:
+        patterns:
+          - "*"