website: Add 21.0 release announcement

Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Change-Id: I4b6f33d3088208da1ab6227ae7fe46b77d81ed7d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/43925
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