Puma is not used as the default server if thin is available

[ghiculescu]

The readme says:

It is recommended to also run gem install puma, which Sinatra will pick up if available.

This suggests that if puma is installed, it will always be used as the server. But that's not true. To demonstrate:

gem uninstall puma sinatra thin # reset things
gem install puma sinatra thin
ruby myapp.rb

Output:

== Sinatra (v3.0.4) has taken the stage on 4567 for development with backup from Thin

If you uninstall thin, you get:

== Sinatra (v3.0.4) has taken the stage on 4567 for development with backup from Puma

---

A fix would be either to change the docs, or to move this line to the bottom of the servers list so that puma is considered first. I'm happy to do a PR for either, but not sure which behaviour is intended.

[dentarg]

So Thin was dropped in #1627 but then added back in 342758a without the docs being updated.

[jkowens]

I do think we should make Puma the default.

[dentarg]

I'm in favour of both having puma as the default and dropping thin, not sure if both of them warrants a new major version? Right now, Thin would have to be dropped if we release a version of Sinatra that only supports Rack 3.

[dentarg]

Actually, #1794, made Falcon to be preferred over Puma too 😅