Update README specifying minimum payShield firmware version

[kevzhzh]

During a customer test run, they were unable to import a key due to an incompatible firmware version in their PayShield HSM, this was due to an unsupported value in the A8 token https://github.com/aws-samples/samples-for-payment-cryptography-service/blob/main/key-import-export/key_exchange/hsm/payshield/commands.py#L155-L164.

• Specify minimum firmware requirements for scripts
• As we are only supported TDES keys, we could potentially also remove the 'D' token value from the A8 command function

[amitkhanal]

"Key Block Version ID" has been changed to 'B' for B8 command. PR - #45