Upgrade to Java 21

Author: miciek

.github/workflows/ci.yml

@@ -7,11 +7,15 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - name: Checkout
-      uses: actions/checkout@v1
-    - name: Setup Scala
-      uses: olafurpg/setup-scala@v13
+      uses: actions/checkout@v4
+    - name: Setup JDK
+      uses: actions/setup-java@v4
       with:
-        java-version: "[email protected]"
+        distribution: temurin
+        java-version: 21
+        cache: sbt
+    - name: Setup sbt launcher
+      uses: sbt/setup-sbt@v1
     - name: Run tests
       run: sbt test
     - name: Run all examples