Update Migration number to ensure consistency on the documentation [ci skip]

Updated Migration number to 6.0 as there were cases where it did show 5.0 and 5.2 which may lead to confusion on a newcomer reader.

guides/source/active_record_basics.md

@@ -366,7 +366,7 @@ database that Active Record supports using `rake`. Here's a migration that
 creates a table:
 
 ```ruby
-class CreatePublications < ActiveRecord::Migration[5.0]
+class CreatePublications < ActiveRecord::Migration[6.0]
   def change
     create_table :publications do |t|
       t.string :title

guides/source/association_basics.md

@@ -101,7 +101,7 @@ NOTE: `belongs_to` associations _must_ use the singular term. If you used the pl
 The corresponding migration might look like this:
 
 ```ruby
-class CreateBooks < ActiveRecord::Migration[5.0]
+class CreateBooks < ActiveRecord::Migration[6.0]
   def change
     create_table :authors do |t|
       t.string :name
@@ -132,7 +132,7 @@ end
 The corresponding migration might look like this:
 
 ```ruby
-class CreateSuppliers < ActiveRecord::Migration[5.0]
+class CreateSuppliers < ActiveRecord::Migration[6.0]
   def change
     create_table :suppliers do |t|
       t.string :name
@@ -176,7 +176,7 @@ NOTE: The name of the other model is pluralized when declaring a `has_many` asso
 The corresponding migration might look like this:
 
 ```ruby
-class CreateAuthors < ActiveRecord::Migration[5.0]
+class CreateAuthors < ActiveRecord::Migration[6.0]
   def change
     create_table :authors do |t|
       t.string :name
@@ -218,7 +218,7 @@ end
 The corresponding migration might look like this:
 
 ```ruby
-class CreateAppointments < ActiveRecord::Migration[5.0]
+class CreateAppointments < ActiveRecord::Migration[6.0]
   def change
     create_table :physicians do |t|
       t.string :name
@@ -304,7 +304,7 @@ end
 The corresponding migration might look like this:
 
 ```ruby
-class CreateAccountHistories < ActiveRecord::Migration[5.0]
+class CreateAccountHistories < ActiveRecord::Migration[6.0]
   def change
     create_table :suppliers do |t|
       t.string :name
@@ -345,7 +345,7 @@ end
 The corresponding migration might look like this:
 
 ```ruby
-class CreateAssembliesAndParts < ActiveRecord::Migration[5.0]
+class CreateAssembliesAndParts < ActiveRecord::Migration[6.0]
   def change
     create_table :assemblies do |t|
       t.string :name
@@ -384,7 +384,7 @@ end
 The corresponding migration might look like this:
 
 ```ruby
-class CreateSuppliers < ActiveRecord::Migration[5.2]
+class CreateSuppliers < ActiveRecord::Migration[6.0]
   def change
     create_table :suppliers do |t|
       t.string :name
@@ -466,7 +466,7 @@ Similarly, you can retrieve `@product.pictures`.
 If you have an instance of the `Picture` model, you can get to its parent via `@picture.imageable`. To make this work, you need to declare both a foreign key column and a type column in the model that declares the polymorphic interface:
 
 ```ruby
-class CreatePictures < ActiveRecord::Migration[5.2]
+class CreatePictures < ActiveRecord::Migration[6.0]
   def change
     create_table :pictures do |t|
       t.string  :name
@@ -483,7 +483,7 @@ end
 This migration can be simplified by using the `t.references` form:
 
 ```ruby
-class CreatePictures < ActiveRecord::Migration[5.0]
+class CreatePictures < ActiveRecord::Migration[6.0]
   def change
     create_table :pictures do |t|
       t.string :name
@@ -514,7 +514,7 @@ With this setup, you can retrieve `@employee.subordinates` and `@employee.manage
 In your migrations/schema, you will add a references column to the model itself.
 
 ```ruby
-class CreateEmployees < ActiveRecord::Migration[5.0]
+class CreateEmployees < ActiveRecord::Migration[6.0]
   def change
     create_table :employees do |t|
       t.references :manager
@@ -575,7 +575,7 @@ end
 This declaration needs to be backed up by a corresponding foreign key column in the books table. For a brand new table, the migration might look something like this:
 
 ```ruby
-class CreateBooks < ActiveRecord::Migration[5.0]
+class CreateBooks < ActiveRecord::Migration[6.0]
   def change
     create_table :books do |t|
       t.datetime   :published_at
@@ -589,7 +589,7 @@ end
 Whereas for an existing table, it might look like this:
 
 ```ruby
-class AddAuthorToBooks < ActiveRecord::Migration[5.0]
+class AddAuthorToBooks < ActiveRecord::Migration[6.0]
   def change
     add_reference :books, :author
   end
@@ -619,7 +619,7 @@ end
 These need to be backed up by a migration to create the `assemblies_parts` table. This table should be created without a primary key:
 
 ```ruby
-class CreateAssembliesPartsJoinTable < ActiveRecord::Migration[5.2]
+class CreateAssembliesPartsJoinTable < ActiveRecord::Migration[6.0]
   def change
     create_table :assemblies_parts, id: false do |t|
       t.bigint :assembly_id
@@ -637,7 +637,7 @@ We pass `id: false` to `create_table` because that table does not represent a mo
 You can also use the method `create_join_table`
 
 ```ruby
-class CreateAssembliesPartsJoinTable < ActiveRecord::Migration[5.0]
+class CreateAssembliesPartsJoinTable < ActiveRecord::Migration[6.0]
   def change
     create_join_table :assemblies, :parts do |t|
       t.index :assembly_id