Fix api_quickstart to use pm.Data instead of MutableData

[Sridhar-0306]

This PR updates the api_quickstart notebook to replace deprecated
pm.MutableData usage with pm.Data, which is the current recommended
API in PyMC.

This fixes an AttributeError when running the notebook with recent
PyMC versions.

• Notebook follows style guide https://docs.pymc.io/en/latest/contributing/jupyter_style.html
• PR description contains a link to the relevant issue:
  • a tracker one for existing notebooks (tracker issues have the "tracker id" label)
  • or a proposal one for new notebooks
• Check the notebook is not excluded from any pre-commit check: https://github.com/pymc-devs/pymc-examples/blob/main/.pre-commit-config.yaml

Helpful links

• https://github.com/pymc-devs/pymc-examples/blob/main/CONTRIBUTING.md

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[Sridhar-0306]

Hi, just a gentle follow-up in case this slipped through during the holiday period.
Happy to make any adjustments if needed. Thanks!