misc: Update the RELEASE-NOTES.md for v21.0.1.0

Change-Id: Id0c30e79100dfb83e53a16ae6a32bf9af3dac705
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/47080
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
1 file changed