deprecate generate synthetic datasets notebook

[tomjemmett]

• removes the synthetic notebook, now in nhp_data
• adds task to download the synthetic data
• uses new folder path for synthetic data
• removes no longer used extension

following on from The-Strategy-Unit/nhp_data#217

[github-actions]

✅ A new build is available.

You can use the following to use pull the image into your local environment:

docker pull ghcr.io/the-strategy-unit/nhp_model:pr-498

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (4f50474) to head (c1aedc6).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #498   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           19        19           
  Lines         1033      1033           
  Branches        50        50           
=========================================
  Hits          1033      1033           

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