Update Jupyter notebooks default kernel name

[ercius]

The kernel name has changed from stempy-jupyterlab to stempy-ipykernel. Please update the templates to use the new kernel name.

For example this line is now incorrect.

[swelborn]

Hey @ercius - we push both tags (stempy-jupyterlab/stempy-ipykernel) for the same image (see https://hub.docker.com/r/openchemistry/stempy-jupyterlab/tags and https://hub.docker.com/r/openchemistry/stempy-ipykernel/tags have the same sha). This means that you can pull either tag and you will get the same image. Maybe last time we just pulled down the wrong tag?

[ercius]

I talked with Chris about this last week. We wanted to reduce the confusion for this and decided to only use stempy-ipykernel. I did not expect this downstream consequence.