Gradle:

- always use Java toolchains
- default is Java 11 (but source/targetCompatibility is still 1.8)
- use system property `toolchain` (e.g. `-Dtoolchain=25`) to compile with other Java versions

preparation for Gradle 9.x, which requires Java 17+ to run

flatlaf-testing/flatlaf-testing-modular-app/build.gradle.kts

@@ -16,6 +16,7 @@
 
 plugins {
 	`java-library`
+	`flatlaf-toolchain`
 	`flatlaf-module-info`
 }