Use Java 21 for downstream testing

Author: gsmet

.github/workflows/downstream.yml

@@ -47,11 +47,11 @@ jobs:
     steps:
     - uses: actions/checkout@v4
 
-    - name: Set up JDK 17
+    - name: Set up JDK 21
       uses: actions/setup-java@v4
       with:
         distribution: 'temurin'
-        java-version: '17'
+        java-version: '21'
 
     - name: Get Date
       id: get-date