Update notebooks to remove deprecated methods

[rosteen]

Recent Jdaviz releases have deprecated (and finally removed) the app.get_data_from_viewer, app.get_subsets_from_viewer, and specviz.load_spectrum methods. This PR updates affected notebooks accordingly. I tagged a few relevant folks as reviewers, wasn't sure who all would want to see this.

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[kecnry]

print(cubeviz.app.data_collection)

I wonder if we need something more user-friendly at the helper level for this case (exploring what data exists in the app, rather than just extracting data where you already know the label, etc)? With what exists now, this probably makes sense for the notebook, though.

[rosteen]

The notebook execution failures are due to needing UI interactions to define subsets before running those notebook cells, I assume those tests have never passed. And the pep8 failures appear unrelated to my changes.

[camipacifici]

The ifu_optimal notebook is in another PR with edits from Patrick. Maybe Ricky's changes have already been addressed in Patrick's PR.

[rosteen]

@haticekaratay I fixed the conflicts with main, I think this is good to go now. I left in two updates to Patrick's IFU notebook to better use current API methods.

[haticekaratay]

Thanks, Ricky, for your work. A follow-up ticket is created to fix the execution and styles that are out of the scope of this PR.