Deprecating web server in favor of language server

[castwide]

In an upcoming version of the gem, I'm looking into removing the legacy web server (solargraph server) and requiring clients to use the language server (solargraph socket). I planned to continue supporting the web server, but removing it has a few important benefits:

1. Ensuring a common, well-documented interface for clients
2. Reducing the number of gem dependencies
3. Making it possible to support Ruby versions older than 2.2.2

I don't mind continuing to support the web server if it's still useful to some people, but in that case it would probably need to be moved into its own gem.

[castwide]

The no-rack-sinatra branch demonstrates the gem without the legacy server. It currently passes tests back to Ruby 2.1.

[castwide]

The legacy web server is removed as of 0.21.0. The minimum Ruby requirement has been moved back to 2.1.

[castwide]

The solargraph gem will proceed with LSP. If anyone would benefit from continuing support for the legacy web server, please feel free to open a new issue.