ci: build entire project on JDK 25

The 21/25 split did not work: Gradle's toolchain auto-detection does not
pick up the JDK 25 that setup-java installs as a secondary version, so
':minestom:compileJava' still could not resolve the Java 25 Minestom
dependency. Run the whole build on JDK 25, consistent with the Titan,
Butterfly and shared OneLiteFeather workflows.

https://claude.ai/code/session_01S76fk9ma5Szkf9r1W44RVP

Author: claude

.github/workflows/ci.yml

@@ -28,9 +28,7 @@ jobs:
         uses: actions/setup-java@v4
         with:
           distribution: 'temurin'
-          java-version: |
-            25
-            21
+          java-version: '25'
 
       - name: Setup Gradle
         uses: gradle/actions/setup-gradle@v4