Add jdk21 to build matrix (#1027)

Author: philsttr

.github/workflows/build.yml

@@ -11,7 +11,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        java: [ '11', '17' ]
+        java: [ '11', '17', '21' ]
     steps:
       - name: Checkout Code
         uses: actions/checkout@v3
@@ -48,7 +48,7 @@ jobs:
 
       - name: Upload Test Coverage to Codecov
         uses: codecov/codecov-action@v3
-        if: matrix.java == '17'
+        if: matrix.java == '21'
         with:
           files: target/site/jacoco/jacoco.xml