Move to a v2-compatible HTTP implementation

[Crell]

This library currently depends on ringcentral/psr7. However, that appears to be abandoned, and only supports PSR-7 v1. Just about everything else has moved to PSR-7 v2, which adds types.

This causes a problem when this package is a transitive dependency, as it means an application using it must use psr/http-message v1, not v2. But some libraries now require v2, making them mutually incompatible.

This is causing issues upstream. See:

https://github.com/reactphp/http/pull/545 https://github.com/reactphp/http/pull/542

Changing the dependency to the more widely used nyholm/psr7 (which supports v2) would resolve this chain of dependencies, and allow React HTTP to be used alongside PSR-7 v2.