Update jupyter-scheduler version in nebari-docker-images

[marcelovilla]

Context

We're currently pinning jupyter-scheduler version to be lower than 2.6.0 to avoid https://github.com/jupyter-server/jupyter-scheduler/issues/519. Since then, a new version has been released, addressing that particular issue: v2.7.0.

We just need to make sure the DB migration is not breaking anything on our side.

Value and/or benefit

Keep up-to-date with newer versions of our dependencies.

Anything else?

I'm tagging @krassowski and @viniciusdc to keep them in the loop.