Update JDK version

Author: mernst

docs/developer/release/README-release-process.html

@@ -237,7 +237,7 @@ <h2 id="continuous-integration">Continuous integration tests</h2>
 
   <ul>
     <li>Checker Framework: <a href="https://dev.azure.com/checkerframework/checkerframework/_build/latest?definitionId=5&branchName=master"><img src="https://dev.azure.com/checkerframework/checkerframework/_apis/build/status%2Ftypetools.checker-framework?branchName=master" alt="Azure Pipelines typetools/checker-framework status"></a></li>
-    <li>JDK: <a href="https://dev.azure.com/checkerframework/checkerframework/_build/latest?definitionId=6&branchName=master"><img src="https://dev.azure.com/checkerframework/checkerframework/_apis/build/status%2Ftypetools.jdk17u?branchName=master" alt="Azure Pipelines typetools/jdk17u status"></a></li>
+    <li>JDK: <a href="https://dev.azure.com/checkerframework/checkerframework/_build/latest?definitionId=6&branchName=master"><img src="https://dev.azure.com/checkerframework/checkerframework/_apis/build/status%2Ftypetools.jdk21u?branchName=master" alt="Azure Pipelines typetools/jdk21u status"></a></li>
     <li>Daikon: <a href="https://dev.azure.com/mernstdaikon/daikon/_build/latest?definitionId=1&branchName=master"><img src="https://dev.azure.com/mernstdaikon/daikon/_apis/build/status%2Fcodespecs.daikon?branchName=master" alt="codespecs/daikon Azure Pipelines status"></a></li>
   </ul>