ci: Update ReadTheDocs build to Ubuntu 22.04

[matthewfeickert]

Description

Update the ReadTheDocs build image to use ubuntu:22.04

c.f. https://twitter.com/readthedocs/status/1522266722635436041

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Use the Docker image ubuntu:22.04 to run the ReadTheDocs build.
   - c.f. https://twitter.com/readthedocs/status/1522266722635436041

[codecov]

Codecov Report

Merging #1860 (0eab673) into master (20d6128) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1860   +/-   ##
=======================================
  Coverage   98.16%   98.16%           
=======================================
  Files          68       68           
  Lines        4315     4315           
  Branches      725      725           
=======================================
  Hits         4236     4236           
  Misses         46       46           
  Partials       33       33           

Flag	Coverage Δ
contrib	26.44% <ø> (ø)
doctest	60.78% <ø> (ø)
unittests-3.10	96.03% <ø> (ø)
unittests-3.7	96.02% <ø> (ø)

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