[hotfix] Add JDK21 for 1.19-SNAPSHOT

Author: MartijnVisser

.github/workflows/weekly.yml

@@ -35,7 +35,7 @@ jobs:
           branch: main
         }, {
           flink: 1.19-SNAPSHOT,
-          jdk: '8, 11, 17',
+          jdk: '8, 11, 17, 21',
           branch: main
         }, {
           flink: 1.17.2,