Get rid of custom logger implementations

[ruudk]

Why does this library ship custom loggers? https://github.com/amplitude/experiment-php-server/tree/main/src/Logger

People can use Monolog or another PSR logger for this.

It would solve the issue we're currently having with psr/log v3 disabled.

@tyiuhc

[kyeh-amp]

@ruudk, I got some time to take a look at your suggestion and just issued #64. The changes require a major release which may seem heavy handed, but I do value your input to steer the SDK closer to psr and industry standards. I'd appreciate your feedback if you have the time!