refactor(root-gradle): deprecated 문법 신규 문법으로 전환

- task -> tasks.register로 전환
- rootProject.buildDir -> rootProject.layout.buildDirectory로 수정

build.gradle

@@ -26,6 +26,6 @@ plugins {
     id 'com.google.firebase.crashlytics' version "2.9.8" apply false
 }
 
-task clean(type: Delete) {
-    delete rootProject.buildDir
+tasks.register('clean', Delete) {
+    delete rootProject.layout.buildDirectory
 }