Update for newer Gradle incremental API

by marking KspTaskJvm.classpathStructure with @incremental. Otherwise
it'd be alwyas treated as unknown changes.

https://docs.gradle.org/7.2/userguide/custom_tasks.html#incremental_tasks
(cherry picked from commit 70f63a8)

gradle-plugin/src/main/kotlin/com/google/devtools/ksp/gradle/KspSubplugin.kt

@@ -41,6 +41,7 @@ import org.gradle.api.tasks.compile.JavaCompile
 import org.gradle.language.jvm.tasks.ProcessResources
 import org.gradle.tooling.provider.model.ToolingModelBuilderRegistry
 import org.gradle.util.GradleVersion
+import org.gradle.work.Incremental
 import org.jetbrains.kotlin.cli.common.arguments.*
 import org.jetbrains.kotlin.gradle.dsl.*
 import org.jetbrains.kotlin.gradle.internal.CompilerArgumentsContributor
@@ -476,6 +477,7 @@ abstract class KspTaskJvm : KotlinCompile(KotlinJvmOptionsImpl()), KspTask {
     @get:PathSensitive(PathSensitivity.NONE)
     @get:Optional
     @get:InputFiles
+    @get:Incremental
     abstract val classpathStructure: ConfigurableFileCollection
 
     @get:Input