Add backwards compatibility item to test checklist

[viniciusdc]

Changes introduced in this PR:

• Add a new item to the testing checklist to validate if the current version of qhub loads the previous YAML config and if the required upgrade config was added.

Types of changes

What types of changes does your PR 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): issue templating update

Further comments (optional)

This might require an extra issue for further discussion as to how we should keep track of the necessary changes required to include in the upgrade command.

[iameskild]

Looks good to me! Thanks @viniciusdc :)