Build with Java 21

I will never not forget this

.github/workflows/build.yml

@@ -3,17 +3,17 @@ name: Java CI with Gradle
 on: [push]
 jobs:
 
-  jdk17:
+  jdk21:
     runs-on: ubuntu-latest
 
     steps:
       - name: Checkout repository
         uses: actions/checkout@v2
 
-      - name: Setup JDK 17
+      - name: Setup JDK 21
         uses: actions/setup-java@v2
         with:
-          java-version: '17'
+          java-version: '21'
           distribution: 'temurin'
 
       - name: Grant execute permission for gradlew