fix: pin Docker.jupyter to notebook 6.x.x

[devstein]

Description

Closes https://github.com/jupyter-incubator/sparkmagic/issues/825

This is a workaround fix. In future we will want to upgrade the Dockerfile to support notebook>=7.0.0

Checklist

• [x] Wrote a description of my changes above
• [x] Formatted my code with black
• [x] Added a bullet point for my changes to the top of the CHANGELOG.md file
• [x] Added or modified unit tests to reflect my changes
• [x] Manually tested with a notebook
• [x] If adding a feature, there is an example notebook and/or documentation in the README.md file