Update build script to use java 15

Author: tkohegyi

.github/workflows/main.yml

@@ -23,10 +23,10 @@ jobs:
       - uses: actions/checkout@v2
         with:
           fetch-depth: 0  # Shallow clones should be disabled for a better relevancy of analysis
-      - name: Set up JDK 11
+      - name: Set up JDK 15
         uses: actions/setup-java@v1
         with:
-          java-version: 11
+          java-version: 15
       - name: Cache SonarCloud packages
         uses: actions/cache@v1
         with: