Before this change CaptureOutput could trip the race detector when
used concurrently. In particular if go routines using the logging are
outlasting the return from `fun()`.
This fixes the problem with a mutex.
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.
Before this change, if rclone is used as a library and logrus is used
after a call to rc `sync/bisync`, logging does not work anymore and
leads to writing to a closed pipe.
This change restores the output correctly.
Fixes#8158
