misc: Update REALEASE-NOTES.md for v21.2.1.0
misc: Update REALEASE-NOTES.md for v21.2.1.0

Change-Id: Ib8daff60c624d8e86f0af872c04b43cb2329a043
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/56250
Reviewed-by: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Maintainer: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5