fix: Bump realm-gradle-plugin to 6.1.0 and fix build

Author: rob729

app/build.gradle

@@ -109,7 +109,7 @@ spotless {
 
 repositories {
     maven { url "https://jitpack.io" }
-    mavenCentral()
+    jcenter()
 }
 
 androidExtensions {

build.gradle

@@ -9,7 +9,7 @@ buildscript {
         classpath "com.android.tools.build:gradle:3.5.3"
         classpath "com.jfrog.bintray.gradle:gradle-bintray-plugin:1.8.4"
         classpath "com.github.dcendents:android-maven-gradle-plugin:2.1"
-        classpath "io.realm:realm-gradle-plugin:5.14.0"
+        classpath "io.realm:realm-gradle-plugin:6.1.0"
         classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:1.3.61"
 
         // NOTE: Do not place your application dependencies here; they belong