misc: Update version number to v21.0.0.0

Change-Id: Ica7f7bfdcb1655e38defdf0b32a630e90adb7ce8
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/43405
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>