Update GitHub CI runners

Change-Id: I16ee17a5c3110f478a36ad2e8861d310a513a085
Reviewed-on: https://korap.ids-mannheim.de/gerrit/c/KorAP/Koral/+/10357
diff --git a/.github/workflows/linux_ci.yml b/.github/workflows/linux_ci.yml
index dd66faf..38062f1 100644
--- a/.github/workflows/linux_ci.yml
+++ b/.github/workflows/linux_ci.yml
@@ -8,11 +8,11 @@
     steps:
       - name: Checkout Koral
         uses: actions/checkout@v4
-      - name: Set up JDK 17
+      - name: Set up JDK 21
         uses: actions/setup-java@v4
         with:
           distribution: 'temurin'
-          java-version: 17
+          java-version: 21
 
       - name: Build and install Koral
         run: mvn --file pom.xml install
diff --git a/.github/workflows/windows_ci.yml b/.github/workflows/windows_ci.yml
index 15acf7d..6e0f94b 100644
--- a/.github/workflows/windows_ci.yml
+++ b/.github/workflows/windows_ci.yml
@@ -12,11 +12,11 @@
           git config --global core.eol lf
       - name: Checkout Koral
         uses: actions/checkout@v4
-      - name: Set up JDK 17
+      - name: Set up JDK 21
         uses: actions/setup-java@v4
         with:
           distribution: 'temurin'
-          java-version: 17
+          java-version: 21
 
       - name: Build and install Koral
         run: mvn --file pom.xml install