Streamline Dependabot configuration

- Remove unnecessary pull request limit, since grouping the updates
  is sufficient to prevent an excessive number of PRs.

- Group GHA updates as well (not just Python ones). This is mostly
  to avoid unnecessary inconsistency, but also sometimes multiple
  related actions get upates at the same time, such that CI jobs
  may not pass if only one is updated.

Author: EliahKagan

.github/dependabot.yml

@@ -6,7 +6,6 @@ updates:
       interval: "monthly"
     allow:
       - dependency-type: "all"
-    open-pull-requests-limit: 20
     groups:
       python-dependencies:
         patterns: ['*']
@@ -15,3 +14,6 @@ updates:
     directory: "/"
     schedule:
       interval: "daily"
+    groups:
+      github-actions:
+        patterns: ['*']