Remove posix runner and unsupported posix-spawn gem

[elisuh]

This PR removes Terrapin's PosixRunner, which relies on the now unsupported posix-spawn gem. Improvements to Ruby (as of Ruby 3.0) means that posix-spawn is now less performant than native Ruby implementation. You can read more about this in this posix-spawn issue, specifically this comment.

We feel it's no longer necessary to provide posix support as there are no longer advantages over native. Since Ruby 2.x will no longer be supported soon, it's important that we maintain Terrapin to be compatible with the lowest supported version of Ruby.

[brian-penguin]

NEAT!

Do you feel up to adding a CI matrix spec setup so we can see in the actions that it runs against all the versions we want to support? (doesn't have to be a part of this PR)

[brian-penguin]

Oh dang looks like you are already on it #16

[elisuh]

Ahhh that was all @joshuaclayton!!! But yea it's done, wanted to put it in a separate PR though since it's such a big change.

[jutonz]

Awesome, I love it! This is so much cleaner. I wonder if we could even just remove BackticksRunner at some point too, since Process.spawn should be available on all currently supported rubies.

I just had one small note about the Runners section of the readme, but other than that this looks great! And since this gem isn't 1.0 yet we don't have to worry about making a breaking change. Great work :)