In preparation for the Ruby 3 upgrade, we should update
json to match the minimum gem version as specified by
the default gem:

https://stdgems.org/json/#gem-version

Which for Ruby 3.0 is 2.5.1.

This will otherwise lead to errors, since rubygems
will encounter conflicting gemspecs during require.

See https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/6286