This removes logrus which is not developed any more and replaces it
with the new log/slog from the Go standard library.
It implements its own slog Handler which is backwards compatible with
all of rclone's previous logging modes.
This changes log statements from log to fs package, which is required for --use-json-log
to properly make log output in JSON format. The recently added custom linting rule,
handled by ruleguard via gocritic via golangci-lint, warns about these and suggests
the alternative. Fixing was therefore basically running "golangci-lint run --fix",
although some manual fixup of mainly imports are necessary following that.
