fix docs.yml

[graebm]

Issue:

the docs.yml job that runs on mainline has been broken for a while

Description of changes:

• Put the required sphinx version in 1 place (requirements-dev.txt)
  • sphinx 7.3+ breaks our docs, not sure why, don't have time to look into it...
• Update docs jobs to run on latest ubuntu22.04
  • sphinx requires python3.9+ now, but ubuntu20.04 has python3.8

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.