[mpp] Set the minimum supported gradle to 6.8.3 (same as in KGP)

KGP supports only Gradle since 6.8.3, and we run tests on
buildserver only since this version, so we shouldn't try to work if
gradle < 6.8.3.
And in general in MPP plugin we shouldn't support version that is not
supported in KGP

#KT-59278

libraries/tools/kotlin-gradle-plugin/src/common/kotlin/org/jetbrains/kotlin/gradle/plugin/mpp/KotlinMultiplatformPlugin.kt

@@ -40,7 +40,7 @@ import org.jetbrains.kotlin.statistics.metrics.StringMetrics
 class KotlinMultiplatformPlugin : Plugin<Project> {
 
     override fun apply(project: Project) {
-        checkGradleCompatibility("the Kotlin Multiplatform plugin", GradleVersion.version("6.0"))
+        checkGradleCompatibility("the Kotlin Multiplatform plugin")
         runDeprecationDiagnostics(project)
 
         project.plugins.apply(JavaBasePlugin::class.java)