Stub out the configuration command and return an error

[adamwg]

Description of your changes

The configuration command doesn't work with Upbound Cloud Spaces or Connected Spaces, and the CLI no longer works with Legacy Spaces, so remove the subcommands from up configuration and print a message.

Fixes #493

I have:

• [x] Read and followed Upbound's contribution process.
• [x] Run make reviewable to ensure this PR is ready for review.
• [ ] Added backport release-x.y labels to auto-backport this PR, as appropriate.

How has this code been tested

Ran the unit tests, and manually verified that up configuration prints the expected message.

[Upbound-CLA]

All committers have signed the CLA.