Make extent not required for VerticalSpatialDimension

[jsignell]

Related Issue(s):

• Closes Datacube VerticalSpatialDimension enforces optional extent #1593

Description:

As @fmigneault points out. This field should not be required on the vertical dimension

PR Checklist:

• Pre-commit hooks pass (run pre-commit run --all-files)
• Tests pass (run pytest)
• Documentation has been updated to reflect changes, if applicable
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.

[codecov]

Codecov Report

❌ Patch coverage is 81.81818% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.29%. Comparing base (aec5ec0) to head (070c2e8).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pystac/extensions/datacube.py	81.81%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1596      +/-   ##
==========================================
- Coverage   92.30%   92.29%   -0.02%     
==========================================
  Files          55       55              
  Lines        8387     8395       +8     
  Branches      970      971       +1     
==========================================
+ Hits         7742     7748       +6     
- Misses        457      458       +1     
- Partials      188      189       +1     

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[fmigneault]

Thanks @jsignell @gadomski