build: compile Minestom modules on Java 25 toolchain

Minestom 2026.05.17-1.21.11 requires a JVM 25+ runtime, so the
relocation-free minestom and minestom:loader modules can no longer be
built against Java 21 (':minestom:compileJava' failed to resolve the
dependency). Switch both modules to a Java 25 toolchain and install
JDK 25 alongside 21 in CI; JDK 21 stays the default JAVA_HOME so the
Forge/Fabric/NeoForge platform builds are unaffected and only the
Minestom modules use the 25 toolchain.

https://claude.ai/code/session_01S76fk9ma5Szkf9r1W44RVP

Author: claude

.github/workflows/ci.yml

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

minestom/build.gradle

@@ -15,8 +15,9 @@ dependencies {
 }
 
 java {
-    sourceCompatibility = JavaVersion.VERSION_21
-    targetCompatibility = JavaVersion.VERSION_21
+    toolchain {
+        languageVersion = JavaLanguageVersion.of(25)
+    }
 }
 
 // Only used occasionally for deployment - not needed for normal builds.

minestom/loader/build.gradle

@@ -16,8 +16,9 @@ dependencies {
 }
 
 java {
-    sourceCompatibility = JavaVersion.VERSION_21
-    targetCompatibility = JavaVersion.VERSION_21
+    toolchain {
+        languageVersion = JavaLanguageVersion.of(25)
+    }
 }
 
 processResources {