Remove deprecated `PrefixedLogger` interface (#4705)
* Add some tests for `logger`
* Remove deprecated `PrefixedLogger` interface
`PrefixedLogger` has been deprecated for some time, so let's remove it now,
while we have a major version bump.
We can tidy up some of the other logic while we're here.
Unfortunately lots of the code still uses `logger.log` which isn't exposed by
the `Logger` interface, so we need to keep exposing that where it was before.
