Refactor the sync workflow of Gemfile

Related Upstream comment: https://gitlab.com/gitlab-org/gitlab/-/issues/415832#note_1446986695

Current workflow see doc: jh/doc/development/backend_guide/add_new_gem.md

After optimization:

• Reuse upstream Gemfile, instead of copy file content;
• Remove useless jh/Gemfile.jh;
• Remove useless script jh/bin/build_static_gemfile;
• Simplify handling of lock files;

cc @mtan-gitlab-cn