[21309] Change back Ubuntu versions for CI and RTD

[EduPonz]

Description

#837 Moved both the CI and RTD generation to Ubuntu 24.04. Although it is a change we want to make, right now both Fast DDS and Fast DDS Python and having warnings and build errors on that platform which are uncovered by:

• [21578] Fix master build to run on Ubuntu 24.04 #856

Until those are fixed, this PR sets the Ubuntu versions back to 22.04 for CI and 20.04.

Contributor Checklist

• Commit messages follow the project guidelines.
• N/A: Code snippets related to the added documentation have been provided.
• N/A: Documentation tests pass locally.
• N/A: Applicable backports have been included in the description.

Reviewer Checklist

• The PR has a milestone assigned.
• The title and description correctly express the PR's purpose.
• Check contributor checklist is correct.
• CI passes without warnings or errors.