Update MvGaussianRandomWalk notebook to v5

[fonnesbeck]

There are some spurious-looking warnings when sampling the random walk model, if anyone cares to take a look.

Also, do we know who the original author of this notebook is?

• [x] Notebook follows style guide https://docs.pymc.io/en/latest/contributing/jupyter_style.html
• [x] PR description contains a link to the relevant issue:
  • https://github.com/pymc-devs/pymc-examples/issues/140
• [x] Check the notebook is not excluded from any pre-commit check: https://github.com/pymc-devs/pymc-examples/blob/main/.pre-commit-config.yaml

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[fonnesbeck]

Should be fixed (not sure why the time stamp is messed up)