Use stable Kotlin compiler frontend (#1506)

* Use stable Kotlin compiler frontend

Using K2 means library consumers must also
update, otherwise they will have errors like
"<classname> is compiled by the new Kotlin
compiler frontend and cannot be loaded by the old
compiler"

* Update changelog

Co-authored-by: paul-dingemans <paul-dingemans@users.noreply.github.com>

CHANGELOG.md

@@ -8,6 +8,8 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 
 ### Fixed
 
+* Remove experimental flag `-Xuse-k2` as it forces API Consumers to compile their projects with this same flag ([#1506](https://github.com/pinterest/ktlint/issues/1506)).
+
 ### Changed
 
 ### Removed

buildSrc/src/main/kotlin/ktlint-kotlin-common.gradle.kts

@@ -19,11 +19,4 @@ kotlin {
     }
 }
 
-tasks.withType<KotlinCompile>().configureEach {
-    kotlinOptions {
-        @Suppress("SuspiciousCollectionReassignment")
-        freeCompilerArgs += listOf("-Xuse-k2")
-    }
-}
-
 addAdditionalJdkVersionTests()