Correctly annotate Gradle task in- & outputs and enable stricter plugin validation (#21957)

* Correctly apply Gradle task input & output annotations to accessors

Gradle's task input and output annotations should be applied to the accessors instead of the properties. No adhering to this can cause funky behavior in the generated stubs. Examples of this can be found in the docs: https://docs.gradle.org/current/userguide/implementing_custom_tasks.html

Also adds missing annotations so that validation passes

* Enable strict Gradle plugin validation

Author: TheMrMilchmann

modules/openapi-generator-gradle-plugin/build.gradle

@@ -81,6 +81,10 @@ tasks.withType(ReleaseNexusStagingRepository).configureEach {
     onlyIf { nexusPublishing.useStaging.get() }
 }
 
+tasks.named("validatePlugins").configure {
+    enableStricterValidation = true
+}
+
 gradlePlugin {
     website = "https://openapi-generator.tech/"
     vcsUrl = "https://github.com/OpenAPITools/openapi-generator"