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

buildSrc/src/main/kotlin/flatlaf-java9.gradle.kts

@@ -18,7 +18,7 @@ plugins {
 	java
 }
 
-if( JavaVersion.current() >= JavaVersion.VERSION_1_9 ) {
+if( java.toolchain.languageVersion.get().asInt() >= 9 ) {
 	sourceSets {
 		create( "java9" ) {
 			java {

buildSrc/src/main/kotlin/flatlaf-module-info.gradle.kts

@@ -29,7 +29,7 @@ plugins {
 	java
 }
 
-if( JavaVersion.current() >= JavaVersion.VERSION_1_9 ) {
+if( java.toolchain.languageVersion.get().asInt() >= 9 ) {
 	sourceSets {
 		create( "module-info" ) {
 			java {

buildSrc/src/main/kotlin/flatlaf-toolchain.gradle.kts

@@ -18,9 +18,6 @@ plugins {
 	java
 }
 
-val toolchainJavaVersion = System.getProperty( "toolchain" )
-if( !toolchainJavaVersion.isNullOrEmpty() ) {
-	java.toolchain {
-		languageVersion = JavaLanguageVersion.of( toolchainJavaVersion )
-	}
+java.toolchain {
+	languageVersion = JavaLanguageVersion.of( System.getProperty( "toolchain", "11" ) )
 }