Bump loglevel of issues reported by --check to warn (#8703)
When prettier --check is run by a script that combines the output from
dozens of separate linters, any progress output like “Checking
formatting...” and “All matched files use Prettier code style!” is
just noise that makes it harder to see whether there are real
errors. Bump the loglevel of issues reported by --check from log to
warn so that the noise can be selectively silenced with
--loglevel=warn.
Signed-off-by: Anders Kaseorg <andersk@mit.edu>