Allow installing Bugsnag PSR Logger v2

[imjoehaines]

Goal

This PR allows installing Bugsnag PSR Logger v2, which adds support for PSR Log v3

This is important for Laravel v9 support, as it will be the first version of Laravel to allow PSR Log v3

Testing

To improve the test coverage of this, I've moved our Laravel & Lumen v8 Maze Runner tests to run on PHP 8.0. This allows them to use Bugsnag PSR Logger v2 as it requires PHP 8.0+

This can be seen in the unit tests — on PHP 7.4 Bugsnag PSR Logger v1 is used, but on PHP 8.0 Bugsnag PSR Logger v2 is used