Undo temporary changes for psr/log ^2 support

[Seldaek]

https://github.com/Seldaek/monolog/pull/1587 did some hackery:

• removed rollbar/rollbar from require-dev, this should be rolled back when https://github.com/rollbar/rollbar-php/pull/536 is merged, and rollbar should be removed from phpstan.neon
• inlined psr/log's LoggerInterfaceTest, when https://github.com/php-fig/log-util/pull/2 is merged, the PsrLogCompatTest should be made to use it again.

See also https://github.com/Seldaek/monolog/pull/1589