[20025] Fix branch selection on Github CI

[EduPonz]

Description

This PR:

1. Fixes the fallback branch for the gh docs CI
2. Fixes the selected .repos file for windows gh CI on push events to 2.12.x

Contributor Checklist

• Commit messages follow the project guidelines.
• N/A: The code follows the style guidelines of this project.
• N/A: Tests that thoroughly check the new feature have been added/Regression tests checking the bug and its fix have been added; the added tests pass locally
• N/A: Any new/modified methods have been properly documented using Doxygen.
• N/A: Changes are ABI compatible.
• N/A: Changes are API compatible.
• N/A: New feature has been added to the versions.md file (if applicable).
• N/A: New feature has been documented/Current behavior is correctly described in the documentation.
• N/A: Applicable backports have been included in the description.

Reviewer Checklist

• The PR has a milestone assigned.
• Check contributor checklist is correct.
• Check CI results: changes do not issue any warning.
• Check CI results: failing tests are unrelated with the changes.

[EduPonz]

@richiprosima please test this