Drop temporary Rollbar\Monolog\Handler\RollbarHandler in favor of the merged pull request in the Monolog repo

[ArturMoczulski]

It looks like Monolog maintainers have finally processed our PR to update their RollbarHandler. This means we can drop our temporary solution \Rollbar\Monolog\Handler\RollbarHandler and the monolog dependency from composer.json.

We need to make sure this is backward compatible for users who already applied the \Rollbar\Monolog\Handler\RollbarHandler solution.

[ArturMoczulski]

We are dropping Monolog as a dependency for the library. Rollbar is still usable with both Monolog 1 and 2.0.0-beta (more info here https://docs.rollbar.com/docs/monolog)

If you have been relying on src/Monolog/Handler/RollbarHandler.php (which was temporary solution for Monolog support) and planning to continue to do so, make sure you take care of the Monolog dependency in your project on your own. More info here: https://github.com/rollbar/rollbar-php/blob/master/src/Monolog/Handler/RollbarHandler.php#L13-L20