Update actions/setup-java to v2 (#335)

- Update setup-java to v2 and explicitly use AdoptOpenJDK
- Enable dependabot for github-actions dependencies

.github/dependabot.yml

@@ -0,0 +1,11 @@
+version: 2
+updates:
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: weekly
+    labels:
+      - ci
+      - github-actions
+    commit-message:
+      prefix: ci

.github/workflows/app-test.yaml

@@ -14,8 +14,9 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v2
       - name: Setup Java
-        uses: actions/setup-java@v1
+        uses: actions/setup-java@v2
         with:
+          distribution: 'adopt'
           java-version: 11
       - name: Setup Gradle cache
         uses: actions/cache@v2