Add rexml to gemspec for Ruby 3.0
Add rexml to gemspec for Ruby 3.0

rexml is no longer default gem in Ruby 3.0.

When we use activemodel 6.1.0 or greater, it includes rexml in its
gemspec and we don't get errors on it.
https://github.com/rails/rails/pull/40281

Otherwise, we can get a LoadError like the following:

      LoadError:
        cannot load such file -- rexml/document
      # ./lib/mongo_mapper/plugins/serialization.rb:92:in `from_xml'

mongomapper/mongomapper