ami: Update environment setup instruction

The latest version of the Open IoT SDK includes
CMSIS-Pack-Utils which requires the following packages:
csolution (v1.5.0), yq, and jq.

Additionally, as Python 3.9 is now required to build
certain components included in the Open IoT SDK, it
is to be set as the default Python version.

The AMI setup instructions as well as the root level README
have been update to reflect these changes.

Signed-off-by: Hugues Kamba-Mpiana <hugues.kambampiana@arm.com>