misc: Updated the RELEASE-NOTES and version number

Updated the RELEASE-NOTES.md and version number for the v20.1.0.4
hotfix release.

Change-Id: Iaefed86cb176c3adcd66d101ac3155d30528b025
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/41713
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
3 files changed