MONGOID-5476 update 9.0 installation version (#5465)

Author: neilshweky

source/installation.txt

@@ -27,7 +27,7 @@ To install the gem with bundler, include the following in your ``Gemfile``:
 
 .. code-block:: ruby
 
-   gem 'mongoid', '~> 7.3.0'
+   gem 'mongoid', '~> 9.0.0'
 
 Using Mongoid with a New Rails Application
 ==========================================