Migration doc suggestion - chsh

[Kevin Horton]

I'm apparently a slow learner, but once again I forgot to change my login shell back to one supplied by Apple before migrating to Big Sur (I had been using.

Suggestion - add a step to the Migration docs, before the step that removes all ports.  If the user is using a default shell provided by Macports, do a 'chsh -s /bin/bash' (or other Apple supplied shell) before removing all ports.

Thanks for MacPorts.

Kevin