Don't use LoggerChannelInterface

Hi,

The problem:
Search API accepts a Logger instance witch is restricted to \Drupal\Core\Logger\LoggerChannelInterface.
This forbids the use of any other logger, such as Monolog.

Reading class comments in LoggerChannelInterface:

This interface defines the full behavior of the central Drupal logger
facility. However, when writing code that does logging, use the generic
\Psr\Log\LoggerInterface for typehinting instead (you shouldn't need the
methods here).

It appears that methods of that interface are quite internal.

Proposed solution:
Replace \Drupal\Core\Logger\LoggerChannelInterface with \Psr\Log\LoggerInterface.

I guess the testbot should tell me if LoggerChannelInterface methods are effectively used.

The patch attached implements the proposed solution.