Enable Dependabot for GitHub Actions and pre-commit hooks

Group Maven updates into a single PR, matching the other ecosystems.
Conform the Maven commit message title to the others and drop the `java`
label for PRs.

Signed-off-by: Benjamin Gilbert <bgilbert@cs.cmu.edu>

Author: bgilbert

.github/dependabot.yml

@@ -1,9 +1,28 @@
 version: 2
 updates:
-  # Periodically check for Maven plugin updates
+  - package-ecosystem: github-actions
+    directory: /
+    groups:
+      github-actions:
+        patterns: ["*"]
+    labels: ["dependencies"]
+    schedule:
+      interval: monthly
+
   - package-ecosystem: maven
     directory: /
+    groups:
+      maven:
+        patterns: ["*"]
+    labels: ["dependencies"]
+    schedule:
+      interval: monthly
+
+  - package-ecosystem: pre-commit
+    directory: /
+    groups:
+      pre-commit:
+        patterns: ["*"]
+    labels: ["dependencies"]
     schedule:
       interval: monthly
-    commit-message:
-      prefix: maven