Support for psr/log:^2.0

As you say, the difference between psr/log 2 and 3 is trivial.
If I were to do it, I would use the latest CI4.5 patched by myself for psr/log 2.0.
The way to patch it is to fork CI4 and make my own package 
or use "composer patches" https://github.com/cweagans/composer-patches.

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