Only require what is needed from cgi

In ruby 3.5 most of the functionality will require adding cgi as a dependency.

Only escape/unescape functions will remain, which is all that's needed here. So just use that functionality from `erb`.

https://bugs.ruby-lang.org/issues/21258.

Author: Earlopain

lib/simplecov-html.rb

@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 
 require "erb"
-require "cgi"
 require "fileutils"
 require "digest/sha1"
 require "time"

views/source_file.erb

@@ -48,7 +48,7 @@
               <% end %>
             <% end %>
 
-            <code class="ruby"><%= CGI.escapeHTML(line.src.chomp) %></code>
+            <code class="ruby"><%= ERB::Util.html_escape(line.src.chomp) %></code>
           </li>
         </div>
       <% end %>