website: Updated supported gcc versions

gem5 is able to compile and run with gcc version 9.3.0 on Ubuntu 20.04 LTS.
The documentation on the building page of the website was updated to
reflect the change in gcc compatibility, as it was previously
stated that gem5 worked with up to version 7 of gcc.

Change-Id: I29a489bce082b0e5fc467891c9767e02f0bae1bf
Jira: https://gem5.atlassian.net/browse/GEM5-194
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/28207
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: Bobby R. Bruce <bbruce@ucdavis.edu>
1 file changed