Pin `jupyter-scheduler` version to below 2.6.0

[krassowski]

Reference Issues or PRs

Pin version to avoid https://github.com/jupyter-server/jupyter-scheduler/issues/519 as discussed in https://github.com/nebari-dev/argo-jupyter-scheduler/pull/10#issuecomment-2120841561

What does this implement/fix?

Put a x in the boxes that apply

• [x] 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?