Use JDK 21

.github/workflows/build_test.yml

@@ -47,10 +47,10 @@ jobs:
         os: [ubuntu-latest]
     steps:
       - uses: actions/checkout@v4
-      - name: Set up JDK 17
+      - name: Set up JDK 21
         uses: actions/setup-java@v3
         with:
-          java-version: '17'
+          java-version: '21'
           distribution: 'temurin'
       - uses: subosito/flutter-action@v2
         with: