Revert to old name as its not needed to change and keep same in both files

lib/gitlab/asciidoc.rb

@@ -33,7 +33,7 @@ module Gitlab
       end
 
       asciidoc_opts = { safe: :secure,
-                        backend: :html5,
+                        backend: :gitlab_html5,
                         attributes: DEFAULT_ADOC_ATTRS,
                         extensions: extensions }
 

lib/gitlab/asciidoc/html5_converter.rb

@@ -6,7 +6,7 @@ module Gitlab
   module Asciidoc
     class Html5Converter < (Asciidoctor::Converter.for 'html5')
 
-      register_for 'html5'
+      register_for 'gitlab_html5'
 
       def convert_stem(node)
         return super unless node.style.to_sym == :latexmath