test with JDK21 in github workflow

.github/workflows/linux.yaml

@@ -16,7 +16,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        java: [11, 17]
+        java: [11, 17, 21]
         os: [ubuntu-latest]
     steps:
       - name: Check out code

.github/workflows/windows.yaml

@@ -16,7 +16,7 @@ jobs:
     runs-on: windows-latest
     strategy:
       matrix:
-        java: [11, 17]
+        java: [11, 17, 21]
     steps:
       - uses: actions/checkout@v5
       - name: Set up JDK ${{ matrix.java }}