Add Ruby version to Gemfile

Since Bundler 2.4.19, including in Ruby 3.2.3, it's possible to define
the Ruby version in the Gemfile by indicating which file contains the
version [1].

There are at least two practical cases where this is an advantage.

First, people using RVM in development will no longer accidentally run
the application using the wrong Ruby version (which, before these
changes, might happen if they switch to a branch using a different Ruby
version and forget to run `rvm use` or exit the current folder and enter
it again) because they will get an error when trying to do so.

Second, people using services like Heroku no longer need to modify the
Gemfile.

The disadvantage is that, now, every time we update the Ruby version, we
have to remember to run `bundle` so our `Gemfile.lock` gets the new
version.

[1] https://github.com/rubygems/rubygems/releases/tag/bundler-v2.4.19

docs/en/installation/deploying-on-heroku.md

@@ -58,7 +58,7 @@ This tutorial assumes that you have already managed to clone Consul Democracy on
 
   **Remember not to commit the file if you have any sensitive information in it!**
 
-6. To ensure Heroku correctly detects and uses the node.js and ruby versions defined in the project, we need to make the following changes:
+6. To ensure Heroku correctly detects and uses the node.js version defined in the project, we need to make the following changes:
 
   In package.json, add the node.js version:
 
@@ -74,18 +74,6 @@ This tutorial assumes that you have already managed to clone Consul Democracy on
   heroku buildpacks:add heroku/nodejs
   ```
 
-  In _Gemfile_, add the ruby version and run bundle:
-
-  ```Gemfile
-  ruby file: ".ruby-version"
-  ```
-
-  and apply:
-
-  ```bash
-  heroku buildpacks:set heroku/ruby
-  ```
-
 7. You can now push your app using:
 
   ```bash