#6622 Logger refactoring: removed manual overriding of the log level in CLI since it's now dependent of the verbosity level

`Piwik\Log::setLogLevel` is now deprecated and doesn't do anything. Its usages have been removed.
#6622 Logger refactoring: using Monolog!

`Piwik\Log` is now a wrapper on top of Monolog. The real logger can now be injected using `Psr\Log\LoggerInterface`.
#6622 Logger refactoring: support for the PSR-3 placeholders in log messages
#6622 Logger refactoring: simplified the formatters by removing the ErrorTextFormatter
#6622 Logger refactoring: simplified the formatters by merging ExceptionHtmlFormatter and ErrorHtmlFormatter
#6622 Logger refactoring: simplified the code and configuration
#6622 Logger refactoring: config simplification