expand tests to include JDK15

Author: Katsute

.github/workflows/java_ci.yml

@@ -8,15 +8,19 @@ jobs:
   build:
     name: Build
     runs-on: ubuntu-latest
+    strategy:
+      matrix: 
+        jdk: [11, 15]
+      fail-fast: false
     steps:
       - name: 📦 Prepate Git Repo
         uses: actions/checkout@v2
 
       - name: ☕ Install OpenJDK
         uses: AdoptOpenJDK/install-jdk@v1
         with:
-          version: '11'
+          version: '${{ matrix.jdk }}'
           architecture: x64
 
       - name: ✔ Test with Maven
-        run: mvn test -fae
+        run: mvn test -fae