Set Fallback branch to 2.6.x

[Mario-DL]

Description

Fix for Shapes Demo fallback branch to 2.6.x, quick fix for #4847

Contributor Checklist

• NO Commit messages follow the project guidelines.
• NO The code follows the style guidelines of this project.
• NO 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
• NO Any new/modified methods have been properly documented using Doxygen.
• NO Any new configuration API has an equivalent XML API (with the corresponding XSD extension)
• NO Changes are backport compatible: they do NOT break ABI nor change library core behavior.
• NO Changes are API compatible.
• NO New feature has been added to the versions.md file (if applicable).
• NO New feature has been documented/Current behavior is correctly described in the documentation.
• NO 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.
• Check CI results: changes do not issue any warning.
• Check CI results: failing tests are unrelated with the changes.

[Mario-DL]

Manual run here https://github.com/eProsima/Fast-DDS/actions/runs/9281413488