refactor dependencies

build.gradle

@@ -1,12 +1,14 @@
 buildscript {
+    def gradle = "7.1.2"
+    def kotlin = "1.6.10"
     repositories {
         google()
         mavenCentral()
         maven { url "https://jitpack.io" }
 
         dependencies {
-            classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:1.6.10"
-            classpath "com.android.tools.build:gradle:7.1.1"
+            classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin"
+            classpath "com.android.tools.build:gradle:$gradle"
         }
     }
 }