fix(model-api-gen-gradle): replace deprecated Project.buildDir usage in Gradle

see https://docs.gradle.org/current/userguide/upgrading_version_8.html#deprecations_2

Author: Oleksandr Dzhychko

docs/global/modules/core/pages/howto/usage-model-api-gen-gradle.adoc

@@ -77,7 +77,7 @@ val mpsVersion: String by rootProject
 val modelixCoreVersion: String by rootProject
 
 // ensure that the MPS directory name is correct
-val mpsDir = buildDir.resolve("mps")
+val mpsDir = project.layout.buildDirectory.dir("mps").get().asFile
 
 val mps by configurations.creating
 val mpsDependencies by configurations.creating

model-api-gen-gradle/src/main/kotlin/org/modelix/metamodel/gradle/MetaModelGradlePlugin.kt

@@ -17,7 +17,7 @@ class MetaModelGradlePlugin : Plugin<Project> {
     override fun apply(project: Project) {
         this.project = project
         this.settings = project.extensions.create("metamodel", MetaModelGradleSettings::class.java)
-        this.buildDir = project.buildDir
+        this.buildDir = project.layout.buildDirectory.get().asFile
 
         val exporterDependencies = project.configurations.create("metamodel-mps-dependencies")
         val exporterDir = getBuildOutputDir().resolve("mpsExporter")