add JDK 21 to GH action build

Signed-off-by: Lukas Jungmann <lukas.jungmann@oracle.com>

.github/workflows/pr.yml

@@ -1,5 +1,5 @@
 #
-# Copyright (c) 2021 Contributors to the Eclipse Foundation
+# Copyright (c) 2021, 2023 Contributors to the Eclipse Foundation
 #
 # This program and the accompanying materials are made available under the
 # terms of the Eclipse Public License v. 2.0 which is available at
@@ -16,6 +16,10 @@ on:
   pull_request:
   push:
 
+concurrency:
+  group: ${{ github.ref }}
+  cancel-in-progress: true
+
 jobs:
   build:
     name: Test on JDK ${{ matrix.java_version }} 
@@ -25,14 +29,9 @@ jobs:
 
     strategy:
       matrix:
-        java_version: [ 11, 17 ]
+        java_version: [ 11, 17, 21-ea ]
 
     steps:
-    - name: Cancel previous runs of this workflow
-      uses: styfle/cancel-workflow-action@0.9.1
-      with:
-        all_but_latest: true
-        access_token: ${{ github.token }}
     - name: Checkout for build
       uses: actions/checkout@v3
     - name: Set up JDK