misc: Updated the RELEASE-NOTES and version number

Updated the RELEASE-NOTES.md and version number for the v20.1.0.5 hotfix
release.

Change-Id: I137a12325137799b9b1f98fe67ac55bfab49cd91
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/43145
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
3 files changed