Add Gradle to dependabot configuration

Dependabot was only creating PRs to update the Maven pom.xml files with
new dependency versions, while the Gradle build.gradle files had to be
updated manually, which was often missed, leaving the Gradle build out
of sync.

Add gradle to the dependabot configuration so that it creates PRs to
update the Gradle build.gradle files as well.

Author: robert-s-ubi

.github/dependabot.yml

@@ -8,6 +8,11 @@ updates:
     schedule:
       interval: "daily"
 
+  - package-ecosystem: "gradle"
+    directory: "/"
+    schedule:
+      interval: "daily"
+
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule: