No need to consider older RubyGem versions since Rails main requires Ruby 3.2.0

Ruby 3.2.0 installs RubyGems 3.4.1 as a default gem.
We can always use the `Gem.ruby_version` now.

* Ruby 3.2.0 Released
https://www.ruby-lang.org/en/news/2022/12/25/ruby-3-2-0-released/
> The following default gems are updated.
> RubyGems 3.4.1

- RubyGems version shipped with Ruby 3.2.0
```ruby
$ ruby -v
ruby 3.2.0 (2022-12-25 revision a528908271) [x86_64-linux]
$ gem -v
3.4.1
```

Refer to
rails#47584
rails#47669

Author: yahonda

railties/lib/rails/generators/base.rb

@@ -423,10 +423,6 @@ def self.default_generator_root # :doc:
           path = File.expand_path(File.join(base_name, generator_name), base_root)
           path if File.exist?(path)
         end
-
-        def gem_ruby_version
-          Gem::Version.new(Gem::VERSION) >= Gem::Version.new("3.3.13") ? Gem.ruby_version : RUBY_VERSION
-        end
     end
   end
 end

railties/lib/rails/generators/rails/app/templates/Dockerfile.tt

@@ -8,7 +8,7 @@
 # For a containerized dev environment, see Dev Containers: https://guides.rubyonrails.org/getting_started_with_devcontainer.html
 
 # Make sure RUBY_VERSION matches the Ruby version in .ruby-version
-ARG RUBY_VERSION=<%= gem_ruby_version %>
+ARG RUBY_VERSION=<%= Gem.ruby_version %>
 FROM docker.io/library/ruby:$RUBY_VERSION-slim AS base
 
 # Rails app lives here

railties/lib/rails/generators/rails/devcontainer/templates/devcontainer/Dockerfile.tt

@@ -1,3 +1,3 @@
 # Make sure RUBY_VERSION matches the Ruby version in .ruby-version
-ARG RUBY_VERSION=<%= gem_ruby_version %>
+ARG RUBY_VERSION=<%= Gem.ruby_version %>
 FROM ghcr.io/rails/devcontainer/images/ruby:$RUBY_VERSION

railties/test/generators/generators_test_helper.rb

@@ -151,7 +151,6 @@ def run_app_update(app_root = destination_root, flags: "--force")
   private
     def gemfile_locals
       {
-        gem_ruby_version: RUBY_VERSION,
         rails_prerelease: false,
         skip_active_storage: true,
         depend_on_bootsnap: false,