qhub upgrade custom auth plus tests

[danlester]

Fixes #931 - qhub upgrade should reject custom authenticators [bug]

Changes:

• Running qhub upgrade will throw an error advising that custom authenticators are no longer supported.
• Users will be told they can run qhub upgrade --attempt-fixes instead. This will change the qhub-config.yaml to the basic Keycloak password authentication.
• Pytests added for upgrade command, including 'normal yaml from v3.10' and custom authenticator (resulting in error, or being fixed by --attempt-fixes)

Types of changes

What types of changes does your code introduce?

Put an x in the boxes that apply

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds a feature)
• Breaking change (fix or feature that would cause existing features to not work as expected)
• Documentation Update
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Build related changes
• Other (please describe):

Testing

Requires testing

• Yes
• No

In case you checked yes, did you write tests?

• Yes
• No

Further comments (optional)

I decided not to pollute documentation with a description of the --attempt-fixes flag since that is self-documenting - it reveals itself if it is needed, and will ideally give the user a chance to understand and think about the pros and cons first.

In pytests for test_upgrade.py, the YAML loading would ideally be a fixture, but maybe not worth changing.