gh-91349: Adjust default compression level to 6 (down from 9) in gzip and tarfile (GH-131470)

gh-91349: Adjust default compression level to 6 (down from 9) in gzip and tarfile

It is the default level used by most compression tools and a
better tradeoff between speed and performance.

Co-authored-by: rmorotti <romain.morotti@man.com>
Co-authored-by: Pieter Eendebak <pieter.eendebak@gmail.com>