unicorn 6.0.0 - no more recycling Rack env

This release allocates a new Rack `env' hash for every request.
This is done for safety with internally-(thread|event)-using Rack
apps which expect to use `env' after the normal Rack response is
complete, but without relying on rack.hijack[1].  Thanks to
Dirkjan Bussink <d.bussink@gmail.com> for the patch:

  https://yhbt.net/unicorn-public/66A68DD8-83EF-4C7A-80E8-3F1F7AB31670@github.com/

The major version is bumped since:

1) there are performance regressions for some simple Rack apps

2) unsupported 3rd-party monkey patches which previously
   relied on this behavior may be broken (our version of
   OobGC was).

The test suite is also more reliable on multi-core systems
and Ruby 3.x.

[1] thread from 2017 around rack.hijack safety:
    https://yhbt.net/unicorn-public/CAAtdryPG3nLuyo0jxfYW1YHu1Q+ZpkLkd4KdWC8vA46B5haZxw@mail.gmail.com/