gem5 version 21.2.0.0

misc: Add release notes for v21.2

Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Change-Id: Ia92440b3b2bcd777b75b0c65ab65252b27734ebb
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/54603
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>