Fix false positive on Java/Kotlin JVM target validation.

Fix false positive case when java toolchain was set to <JDK1.8. In such
case Java tasks return '8' (or less) for target instead of '1.8'.

^KT-47520 Fixed

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/KotlinJavaToolchainTest.kt

@@ -590,6 +590,37 @@ class KotlinJavaToolchainTest : KGPBaseTest() {
         }
     }
 
+    @DisplayName("Should correctly validate JVM targets in mixed Kotlin/Java projects that are using <JDK1.8")
+    @GradleTestVersions(minVersion = "6.7.1")
+    @GradleTest
+    internal fun oldJdkMixedJavaKotlinTargetVerification(gradleVersion: GradleVersion) {
+        project(
+            projectName = "kotlinJavaProject".fullProjectName,
+            gradleVersion = gradleVersion
+        ) {
+            //language=groovy
+            rootBuildGradle.append(
+                """
+                
+                java {
+                    toolchain {
+                        languageVersion.set(JavaLanguageVersion.of(8))
+                    }
+                }
+                
+                """.trimIndent()
+            )
+            //language=properties
+            gradleProperties.append(
+                """
+                kotlin.jvm.target.validation.mode = error
+                """.trimIndent()
+            )
+
+            build("build")
+        }
+    }
+
     private fun BuildResult.assertJdkHomeIsUsingJdk(
         javaexecPath: String
     ) = assertOutputContains("[KOTLIN] Kotlin compilation 'jdkHome' argument: $javaexecPath")

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/tasks/Tasks.kt

@@ -670,8 +670,15 @@ abstract class KotlinCompile @Inject constructor(
 
     private fun validateKotlinAndJavaHasSameTargetCompatibility(args: K2JVMCompilerArguments) {
         associatedJavaCompileTaskTargetCompatibility.orNull?.let { targetCompatibility ->
-            val normalizedJavaVersion = if (targetCompatibility == "1.9") "9" else targetCompatibility
-            if (normalizedJavaVersion != args.jvmTarget) {
+            val normalizedJavaTarget = when (targetCompatibility) {
+                "6" -> "1.6"
+                "7" -> "1.7"
+                "8" -> "1.8"
+                "1.9" -> "9"
+                else -> targetCompatibility
+            }
+
+            if (normalizedJavaTarget != args.jvmTarget) {
                 val javaTaskName = associatedJavaCompileTaskName.get()
                 val errorMessage = "'$javaTaskName' task (current target is $targetCompatibility) and " +
                         "'$name' task (current target is ${args.jvmTarget}) " +