purge most danlging qhub references

[pmeier]

While only very few, there are a some references to QHub left in the repository. This PR updates most of them. The ones left are

• inside the RELEASE.md file: we obviously can't remove them
• inside nebari/upgrade.py or test/test_upgrade.py: same as above
• inside the docs/ directory: handled by purge docs and nox #1801
• inside tests_e2e/cypress: since test is green now and playwright (Initial playwright setup #1665) will replace the cypress tests soon, I just left it

What does this implement/fix?

• 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 not to work as expected)
• Documentation Update
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Build related changes
• Other (please describe):

Testing

• Did you test the pull request locally?
• Did you add new tests?

[iameskild]

Thanks @pmeier! I made one minor suggested change but otherwise, this can be merged :)