Remove updating of rubygems

[tgxworld]

This adds an additional layer of 30mb for no reason. We don't need to be
running the latest version of rubygems all the time.

tgxworld ~/work/discourse_docker [skip_updating_ruby_gems] $ docker history --no-trunc discourse/base:release | grep 'gem update --system'
<missing>                                                                 5 hours ago    RUN |1 DEBIAN_RELEASE=bookworm /bin/sh -c echo 'gem: --no-document' >> /usr/local/etc/gemrc &&    gem update --system # buildkit                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                29.8MB    buildkit.dockerfile.v0

Even if we need to update rubygems, it should be updated in https://github.com/discourse/docker-ruby

[tgxworld]

Note that the ruby image that this image is based off already does this: https://github.com/discourse/docker-ruby/blob/832c4cbc71ae9ad37b7206d83b73e473472a2e30/Dockerfile#L31-L34