website: Add gem5 v21.2 release blog post

Change-Id: I534ed5ca7de8408c71397b6dd50ec06e79073aa1
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/54963
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: Bobby Bruce <bbruce@ucdavis.edu>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
1 file changed