website: Updated Scons version to 3.0+

The minimum required Scons version for gem5 20.1 is now 3.0. This
patch updates this on the website.

Change-Id: I5680d6aeba845796c74ffb964ab08bdb4086d228
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/35355
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: Bobby R. Bruce <bbruce@ucdavis.edu>
2 files changed