Clean up Kotlin code in buildSrc/ (#1512)

- fix most IntelliJ warnings (regex refactoring was done by IDE)
- replace hardcoded JVM target versions with BuildInfo.jvmTarget

buildSrc/src/main/kotlin/NativeImageBuild.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2025-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -102,6 +102,7 @@ abstract class NativeImageBuild : DefaultTask() {
   }
 
   @TaskAction
+  @Suppress("unused")
   protected fun run() {
     execOperations.exec {
       val exclusions =