readthedocs: use Ubuntu 26.04/Python 3.14

[pjonsson]

Reason for this pull request

Bump the build environment to
the latest Ubuntu LTS and its
Python version.

Proposed changes

• Closes #xxxx

• Tests added / passed

• Pull Request Title will make sense in ODC Release Notes

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📚 Documentation preview 📚: https://opendatacube--2510.org.readthedocs.build/en/2510/

[pjonsson]

The changes in the rendered documentation from this are a couple of formatting changes, some API changes when inheriting from Exception, and that union types are rendered as A | B instead of the previous Union[A, B].

Personally I find the A | B syntax easier to read, so there are benefits of this beyond just moving the build to an Ubuntu LTS version that will be supported for the next 5 years.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.54%. Comparing base (4324a1f) to head (17b14cd).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2510   +/-   ##
========================================
  Coverage    86.54%   86.54%           
========================================
  Files          162      162           
  Lines        17307    17307           
========================================
  Hits         14978    14978           
  Misses        2329     2329           

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