Mark as unmaintained

[andymckay]

Sadly we haven't got the time to maintain this repository as an official GitHub Action. In this case ruby/ruby-setup has become the default go to Action and we use that in our starter workflows.

We'd like to mark this repository as unmaintained and make it clear to users what their expectations should be.

People are of course free to fork this repository and maintain those forks if they like.

To do this we'll:

• Remove it from the Marketplace (wasn't on the Marketplace)
• Remove from any official GitHub documentation (not in the documentation)
• Change the README.md to clearly reflect its new state.
• Update the README.md to point to other community Actions.
• Archive this repository, putting it read-only mode. It will continue to work for all existing Action users, but won't receive any updates.