build: use Java 25 toolchain for minestom:app (Gradle 9)

Gradle 9 removed project-level sourceCompatibility/targetCompatibility,
which broke ':minestom:app' evaluation after the upstream merge. Replace
them with a Java 25 toolchain, consistent with the minestom and
minestom:loader modules.

https://claude.ai/code/session_01S76fk9ma5Szkf9r1W44RVP

Author: claude

minestom/app/build.gradle

@@ -3,8 +3,13 @@ plugins {
     id("java-library")
 }
 group = 'net.luckperms'
-sourceCompatibility = 21
-targetCompatibility = 21
+
+java {
+    toolchain {
+        languageVersion = JavaLanguageVersion.of(25)
+    }
+}
+
 dependencies {
     implementation project(':api')