Add a command for switching core version

[andrewbelcher]

Is there an existing issue for this?

• I have searched the existing issues

Is your feature request related to a problem?

Switching core version means editing .ddev/config.local.yaml and running two commands. A simple command could make that into a CLI one-liner.

Describe your solution

A host command that will write the appropriate env var in config.local.yaml, restart, and run poser.

Describe alternatives

No response

Additional context

No response