ci: run Gradle on JDK 21, expose JDK 25 toolchain via env

Gradle 8.14.3 cannot run on JDK 25 ('Unsupported class file major
version 69'), so the full-JDK-25 runtime broke the build script
compilation. Keep the Gradle runtime on JDK 21 (last entry, so it is
the default JAVA_HOME) and additionally install JDK 25, exposing it to
Gradle toolchain detection via org.gradle.java.installations.fromEnv so
only the Java 25 Minestom modules use it.

https://claude.ai/code/session_01S76fk9ma5Szkf9r1W44RVP

Author: claude

.github/workflows/ci.yml

@@ -28,13 +28,15 @@ jobs:
         uses: actions/setup-java@v4
         with:
           distribution: 'temurin'
-          java-version: '25'
+          java-version: |
+            25
+            21
 
       - name: Setup Gradle
         uses: gradle/actions/setup-gradle@v4
 
       - name: Run build and tests with Gradle wrapper
-        run: ./gradlew test build publish cyclonedxBom -PdockerTests
+        run: ./gradlew test build publish cyclonedxBom -PdockerTests -Dorg.gradle.java.installations.fromEnv=JAVA_HOME_25_X64
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
           NPM_TOKEN: ${{ secrets.NPM_TOKEN }}