Updating for PHP 8.0

[Bob Strecansky]

Description

While using the guzzle6-adapter in opentelemetry-php I noticed that PHP 8.0 is not yet supported. I think the version constraint here might need to be changed to >=7.1, but there might be some additional testing necessary.

Example

You should be able to see this dependency resolution in this github action installation attempt:
https://github.com/bobstrecansky/opentelemetry-php/pull/6/checks?check_run_id=1364216037#step:6:14

[David Buchmann]

are you sure that guzzle version 6 actually works on php 8?
i'd recommend to use guzzle 7 which implements psr-18 natively, or if you need the php-http async interface, use guzzle 7 with guzzle7-adapter

[Bob Strecansky]

I'll give that a go and report back. thanks @dbu !