Revert "Github CI: Build with JDK 15, test with matrix"

This reverts commit cecd035.

Author: mkarg

.github/workflows/maven.yml

@@ -43,18 +43,10 @@ jobs:
             maven-${{ matrix.os }}-java${{ matrix.java }}-
             maven-${{ matrix.os }}-
 
-      - name: Set up JDK for building
+      - name: Set up JDK
         uses: actions/setup-java@v1
         with:
-          java-version: 15
+          java-version: ${{ matrix.java }}
 
       - name: Build with Maven
         run: mvn verify javadoc:javadoc -e -B -V
-
-      - name: Set up JDK for testing
-        uses: actions/setup-java@v1
-        with:
-          java-version: ${{ matrix.java }}
-
-      - name: Test with Maven
-        run: mvn verify -e -B -V