Remove the ipykernel<7 pin once the Jupyter kernel hang is fixed

We pinned ipykernel to 6.x in #3661 as a temporary workaround.

ipykernel 7 can leave notebooks hanging on the first cell, the kernel runs the cell, but VS Code never gets the messages that clear the spinner. Reloading the window or restarting the kernel clears it.

Upstream: https://github.com/microsoft/vscode-jupyter/issues/17481

The pin lives in two places:
- `source/vscode/resources/qdk-learning/courses/chemistry-qpe/requirements.txt`
- `source/vscode/src/pythonEnvs.ts`, the venv package picker, where the label doubles as the pip specifier

Once the upstream fix ships, remove both and go back to installing the latest ipykernel.