Fix compile error.

Author: bennyhuo

trimindent-gradle-plugin/build.gradle.kts

@@ -1,3 +1,5 @@
+import org.jetbrains.kotlin.gradle.dsl.JvmTarget
+
 plugins {
     id("java-gradle-plugin")
     kotlin("jvm")
@@ -18,6 +20,12 @@ buildConfig {
     buildConfigField("String", "KOTLIN_PLUGIN_VERSION", "\"${compilerPluginProject.version}\"")
 }
 
+kotlin {
+    compilerOptions {
+        jvmTarget.set(JvmTarget.JVM_1_8)
+    }
+}
+
 gradlePlugin {
     plugins {
         create("TrimIndentGradlePlugin") {