Short-term updates to facial-recognition-ai-python

[anisa-hawes]

I'm opening this PR to make the following short-term adjustments to facial-recognition-ai-python:

1. Add os.environ["CUDA_VISIBLE_DEVICES"] = "-1" as final line of the Preliminaries block (in .md and .ipynb)
2. Add new note explaining compatibility problem between TensorFlow and Google Colab / disabling GPU
3. Remove existing note advising readers to ensure connection to a GPU runtime

Reference: #3558

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[anisa-hawes]

Hello Alex @hawc2,

As you know, we have been in dialogue with Charlie Goldberg (author of facial-recognition-ai-python) about a reader-reported error with this lesson.

Charlie has advised proceeding with some short-term updates, while offering to work on some broader revisions to ensure the lesson can remain useful into the future.

This PR applies the short-term updates that make the lesson usable as it is (disabling the GPU for the session).