Upgrade kotlin to 1.4.0, junit to 5.6.2 & gradle to 6.6

Kotlin/build.gradle.kts

@@ -0,0 +1,29 @@
+import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
+
+plugins {
+	kotlin("jvm") version "1.4.0"
+}
+
+group = "com.gildedrose"
+version = "1.0-SNAPSHOT"
+
+repositories {
+	mavenCentral()
+}
+
+dependencies {
+	implementation(kotlin("stdlib"))
+	testImplementation("org.junit.jupiter:junit-jupiter:5.6.2")
+}
+
+tasks.test {
+	useJUnitPlatform()
+	testLogging {
+		events("passed", "skipped", "failed")
+	}
+}
+
+// config JVM target to 1.8 for kotlin compilation tasks
+tasks.withType<KotlinCompile>().configureEach {
+	kotlinOptions.jvmTarget = "1.8"
+}