[Fix #12310] Drop base64 gem from runtime dependency

Fixes #12310.

This PR drops `base64` gem from runtime dependency.

RuboCop only uses `Base64.encode64` from the `base64` gem. Therefore, there's no need to depend on
the entire `base64` gem. The implementation of `Base64.encode64` is quite simple:
https://github.com/ruby/base64/blob/v0.1.1/lib/base64.rb#L27-L40

This change is a better approach that has also been adopted by rack/rack#2110.

changelog/fix_drop_base64_gem_from_runtime_dependency.md

@@ -0,0 +1 @@
+* [#12310](https://github.com/rubocop/rubocop/issues/12310): Drop `base64` gem from runtime dependency. ([@koic][])

lib/rubocop/formatter/html_formatter.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 
-require 'base64'
 require 'cgi'
 require 'erb'
 require 'ostruct'
@@ -124,7 +123,10 @@ def escape(string)
 
         def base64_encoded_logo_image
           image = File.read(LOGO_IMAGE_PATH, binmode: true)
-          Base64.encode64(image)
+
+          # `Base64.encode64` compatible:
+          # https://github.com/ruby/base64/blob/v0.1.1/lib/base64.rb#L27-L40
+          [image].pack('m')
         end
       end
     end

rubocop.gemspec

@@ -31,7 +31,6 @@ Gem::Specification.new do |s|
     'rubygems_mfa_required' => 'true'
   }
 
-  s.add_runtime_dependency('base64', '~> 0.1.1')
   s.add_runtime_dependency('json', '~> 2.3')
   s.add_runtime_dependency('language_server-protocol', '>= 3.17.0')
   s.add_runtime_dependency('parallel', '~> 1.10')