Configure dependabot to update Gradle plugins

The test Gradle sample apps reference plugins, and it's good to keep these
updated (particularly the GE plugin).

.github/dependabot.yml

@@ -1,4 +1,10 @@
 version: 2
+registries:
+  gradle-plugin-portal:
+    type: maven-repository
+    url: https://plugins.gradle.org/m2
+    username: dummy # Required by dependabot
+    password: dummy # Required by dependabot
 updates:
   - package-ecosystem: "npm"
     directory: "/"
@@ -6,3 +12,15 @@ updates:
       interval: "daily"
     ignore:
       - dependency-name: "@types/node"
+  - package-ecosystem: "gradle"
+    directory: "__tests__/samples/kotlin-dsl"
+    registries:
+      - gradle-plugin-portal
+    schedule:
+      interval: "daily"
+  - package-ecosystem: "gradle"
+    directory: "__tests__/samples/no-wrapper"
+    registries:
+      - gradle-plugin-portal
+    schedule:
+      interval: "daily"