chore: Update to Java 21

buildSrc/src/main/kotlin/twomartens.kotlin.gradle.kts

@@ -1,4 +1,5 @@
 import org.gradle.accessors.dm.LibrariesForLibs
+import org.jetbrains.kotlin.gradle.dsl.JvmTarget
 
 plugins {
     kotlin("jvm")
@@ -11,8 +12,11 @@ dependencies {
     implementation(kotlin("reflect"))
 }
 
+val projectSourceCompatibility: String = rootProject.properties["projectSourceCompatibility"].toString()
+
 kotlin {
     compilerOptions {
         freeCompilerArgs.add("-Xjvm-default=all")
+        jvmTarget.set(JvmTarget.fromTarget(projectSourceCompatibility))
     }
 }