update billing client to 5.2.1

Author: onkar-rentomojo

playbilling/build.gradle

@@ -56,7 +56,7 @@ dependencies {
     api 'androidx.browser:browser:1.4.0'
 
     implementation fileTree(dir: 'libs', include: ['*.jar'])
-    implementation 'com.android.billingclient:billing:4.0.0'
+    implementation 'com.android.billingclient:billing:5.2.1'
 
     testImplementation 'junit:junit:4.12'
     testImplementation 'org.robolectric:robolectric:4.4'