Allow users to mark gems as no longer being maintained.

[colby-swandale]

Is your feature request related to a problem?

Presently, the RubyGems team will only consider requests to yank a gem if it presents a high risk to users' security. Consequently, users who longer wish to maintain their gems are no longer able to remove themselves of their responsibility as authors.

Describe the solution you'd like

We should:

1. Add an attribute to Rubygem for authors to indicate the gem is no longer being maintained.
2. When enabled, RubyGems.org should indicate the gem is marked as deprecated / no longer maintained.
3. Bundler/RubyGems should warn users on this field during gem install & bundle install

Additional context

#4631