Fix deprecated configuration key for readthedocs

[dhomeier]

Description

The configuration key "build.image" is deprecated.

Unfortunately the replacement os only accepts a specific version and is also non-optional, so I see this break again whenever ubuntu-22.04 should be no longer available.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (0976a06) 86.62% compared to head (3cbeaa9) 86.62%.
Report is 1 commits behind head on main.

❗ Current head 3cbeaa9 differs from pull request most recent head f1a6209. Consider uploading reports for the commit f1a6209 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #405   +/-   ##
=======================================
  Coverage   86.62%   86.62%           
=======================================
  Files          89       89           
  Lines        5130     5130           
=======================================
  Hits         4444     4444           
  Misses        686      686           

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