Use monolog/monolog 3.x for PHP 8.1+

[n7studios]

Summary

Permit using monolog/monolog 3.x to ensure this package works with other packages that require 3.x, such as Laravel 10, as reported here.

PHP < 8.1 will continue to use monolog 2.x, with our tests running in PHP 7.4 and 8.0 confirming this works.

Testing

• testDebugEnabled: Test that the debug log is created and written to with expected data when debugging is enabled
• testDebugDisabled: Test that the debug log is not created when debugging is disabled

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• The code passes when running PHPStan
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)

[jtolj]

@n7studios thanks for getting this updated so quickly!