@input and @internal should not be apply on the same property (#9059)

Motivation:

In Gradle 7.0 apply @input and @internal on property is forbidden.
Before Gradle 7.0 applying both on the same property is confusing. According to the documentation:

* @input: Attached to a task property to indicate that the property specifies some input value for the task.
* @internal: Attached to a task property to indicate that the property is not to be taken into account for up-to-date checking

modules/openapi-generator-gradle-plugin/src/main/kotlin/org/openapitools/generator/gradle/plugin/tasks/GenerateTask.kt

@@ -81,9 +81,8 @@ open class GenerateTask : DefaultTask() {
     val outputDir = project.objects.property<String>()
 
     @Suppress("unused")
-    @get:Internal
     @set:Option(option = "input", description = "The input specification.")
-    @Input
+    @Internal
     var input: String? = null
         set(value) {
             inputSpec.set(value)