Kotlin multiplatform updated to fit future Kotlin 1.6.0 release (OpenAPITools#10468)

* Update Kotlin Version to 1.6.0-M1
Update Multiplatform Plugin to 1.6.0-M1
Update Gragle to 7.2
Update AGP to 7.0.2

Update samples accordingle

Resolve: OpenAPITools#10467

* Removes local.properties from kotlin-multiplatform petstore client, that was accidentally added

* Use the latest kotlin stable version 1.5.31

* update samples

Co-authored-by: William Cheng <wing328hk@gmail.com>

Author: shadowsheep1

modules/openapi-generator/src/main/resources/kotlin-client/libraries/multiplatform/auth/HttpBasicAuth.kt.mustache

@@ -7,7 +7,7 @@ class HttpBasicAuth : Authentication {
     var username: String? = null
     var password: String? = null
 
-    @InternalAPI
+    @OptIn(InternalAPI::class)
     override fun apply(query: MutableMap<String, List<String>>, headers: MutableMap<String, String>) {
         if (username == null && password == null) return
         val str = (username ?: "") + ":" + (password ?: "")

samples/client/petstore/kotlin-multiplatform/gradle/wrapper/gradle-wrapper.properties

@@ -1,5 +1,6 @@
+#Fri Sep 24 10:59:07 CEST 2021
 distributionBase=GRADLE_USER_HOME
+distributionUrl=https\://services.gradle.org/distributions/gradle-7.2-bin.zip
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-6.0-bin.zip
-zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
+zipStoreBase=GRADLE_USER_HOME

samples/client/petstore/kotlin-multiplatform/src/commonMain/kotlin/org/openapitools/client/auth/HttpBasicAuth.kt

@@ -7,7 +7,7 @@ class HttpBasicAuth : Authentication {
     var username: String? = null
     var password: String? = null
 
-    @InternalAPI
+    @OptIn(InternalAPI::class)
     override fun apply(query: MutableMap<String, List<String>>, headers: MutableMap<String, String>) {
         if (username == null && password == null) return
         val str = (username ?: "") + ":" + (password ?: "")