[hotfix] Add Java 21 as JDK to test for Flink 1.19.0 weekly runs

Author: MartijnVisser

.github/workflows/weekly.yml

@@ -51,7 +51,7 @@ jobs:
         }, {
           flink: 1.19.0,
           branch: v3.1,
-          jdk: '8, 11, 17',
+          jdk: '8, 11, 17, 21',
         }]
     uses: apache/flink-connector-shared-utils/.github/workflows/ci.yml@ci_utils
     with: