Verify the "Renku version" message is correct

[lorenzo-cavazzi]

Description

On PR deployments, we occasionally show a "New version" message even when the API response from cache.migrations_check doesn't suggest required actions (see screenshot).
It's possible things change recently, we should double-check if this works fine on our side.

Reproduce

Use a non-tagged version of renku-core. E.G. open a PR and use /deploy renku-core=develop #persist and create a new project from that deployment.

[lorenzo-cavazzi]

More info: we show the message based on (among the others) dockerfile_renku_status.newer_renku_available.
So far, we use this convention:

• true means update
• false means no-update
• null means there might be an update

We should consider null as "we don't have enough information on the project version" and show such a message.
This should not happen in production (unless the project is very old) but it's common on our dev cluster