Add Gem.disable_system_update_message to disable gem update --system if needed.

[simi]

This is initial take on possibility to disable gem update --system useful for distribution maintainers.

For Debian, this can be set at https://salsa.debian.org/ruby-team/rubygems-integration/-/blob/39d3a082f568ec0b6c9acd10e54a4a31a08bd8b5/lib/rubygems/defaults/operating_system.rb#L56. End-users can disable this by providing DEBIAN_DISABLE_RUBYGEMS_INTEGRATION as they can do that already for the rest of integration patches.

For Fedora, this can be set at https://src.fedoraproject.org/rpms/rubygems/blob/master/f/operating_system.rb. Not sure if any env variable is needed to make users able to disable this.

@terceiro (Debian ruby maintainer) @voxik (Fedora ruby maintainer), would this be useful for you ❓

example

# operating_system.rb
if Gem.respond_to?(:disable_system_update_message=)
  Gem.disable_system_update_message = 'RubyGems is updated by system package manager. Please use "abc update rubygems" instead.'
end

[retro@retro  rubygems (disable-update *%=)]❤ gem update --system
ERROR:  RubyGems is updated by system package manager. Please use "abc update rubygems" instead.

[terceiro]

would this be useful for you ?

yes

[voxik]

LGTM

[deivid-rodriguez]

Neat stuff!