[24136] Update fallback branches in conf.py

[MiguelCompany]

Description

Use the branches corresponding to the 3.4.x branch

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: The Pro version badge has been added if the documented feature is exclusive to Fast DDS Pro.
• 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.