Introduce unspecifiedJvmPlatform for clients without jvmTarget

Previously, a lot of clients used JvmPlatform as platform-marker,
without thinking about jvmTarget.

For the sake of migration, this commits introduced so-called
UnspecifiedJvmPlatform, which can be used for a time being, but
generally, all usages should be removed in  future.

idea/idea-maven/src/org/jetbrains/kotlin/idea/maven/configuration/KotlinJavaMavenConfigurator.kt

@@ -68,7 +68,7 @@ class KotlinJavaMavenConfigurator : KotlinMavenConfigurator(
     }
 
     override val targetPlatform: TargetPlatform
-        get() = JvmPlatforms.defaultJvmPlatform
+        get() = JvmPlatforms.unspecifiedJvmPlatform
 
     @Suppress("DEPRECATION_ERROR")
     override fun getTargetPlatform() = JvmPlatforms.CompatJvmPlatform

idea/idea-maven/test/org/jetbrains/kotlin/idea/maven/KotlinMavenImporterTest.kt

@@ -2529,7 +2529,7 @@ class KotlinMavenImporterTest : MavenImportingTestCase() {
         }
 
         with(facetSettings("my-jvm-module")) {
-            Assert.assertEquals(JvmPlatforms.defaultJvmPlatform, platform)
+            Assert.assertEquals(JvmPlatforms.jvm16, platform)
             Assert.assertEquals(listOf("my-common-module1", "my-common-module2"), implementedModuleNames)
         }
 
@@ -2974,8 +2974,8 @@ class KotlinMavenImporterTest : MavenImportingTestCase() {
 
         assertImporterStatePresent()
 
-        checkStableModuleName("project", "project", JvmPlatforms.defaultJvmPlatform, isProduction = true)
-        checkStableModuleName("project", "project", JvmPlatforms.defaultJvmPlatform, isProduction = false)
+        checkStableModuleName("project", "project", JvmPlatforms.unspecifiedJvmPlatform, isProduction = true)
+        checkStableModuleName("project", "project", JvmPlatforms.unspecifiedJvmPlatform, isProduction = false)
     }
 
     fun testStableModuleNameWhileUsngMaven_JS() {