ext,misc: Update `ext/sst/README.md` for v21.2 release
ext,misc: Update `ext/sst/README.md` for v21.2 release

Change-Id: I2cd982c819074962182da43754399852f9e72174
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/54703
Reviewed-by: Bobby Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>
Maintainer: Bobby Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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