Fatal Error when upgrading PSR/Log to v3

[maikelRAOW]

As mentioned, the package can be used with PSR/Log 3 as mentioned in this issue:
#312
While the tickets are closed no action has been taken to resolve this as the composer.json still claims compatibility with PSR/Log 3 as of version 3.1.0.
This makes it extremely easy to upgrade into a fatal error.

[tbourdin]

@maikelRAOW i managed the issue with the beta.
Just need to correct the composer json with : "mautic/api-library": "4.0.0-beta",