website: Update the URLs to point towards the v21.1 bucket

Change-Id: Ia20ebdd976c44a203926505b53c8790133e3558e
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/48320
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: Bobby R. Bruce <bbruce@ucdavis.edu>
5 files changed