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).
Minor changes to the build scripts were required to satisfy Dependabot.

.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"