Deprecate ILogger

The logger service was always intended to follow the PSR-3 interface.
It's time to embrace this and switch over to the "official" API,
hence this custom interface can be slowly phased out.

With Nextcloud 20 the logger also got support for
* App id filled out automatically
* Exceptions handling (as replacement for logException)

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>