[21256] Fix issue with Fast DDS and Fast DDS python deduced branches (backport #838)

[mergify]

Description

In the previous PR

• [21256] Migrate Fast DDS Docs CI to Github #831

The Fast DDS and Fast DDS python related branches were not used as expected:
Instead of using the provided branch, it was downloading the .repos file from that branch (so the base reference was used instead of the desired branch).
This hotfix PR performs a checkout before downloading the repos included in the .repos file, so the actual branch used is the desired one.
In the case of the Fast DDS Python, its dependencies are already downloaded, so there is no need to download them. It has been changed to just cloning that repo and checking out to the desired branch.

Important: when fixing the conflicts in the backports, please ensure:

• Change fastdds.repos to fastrtps.repos
• change src/fastdds to src/fastrtps

@Mergifyio backport 2.14.x 2.13.x 2.10.x 2.6.x

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.
• 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.

---

This is an automatic backport of pull request #838 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 6a53b9e has failed:

On branch mergify/bp/2.14.x/pr-838
Your branch is up to date with 'origin/2.14.x'.

You are currently cherry-picking commit 6a53b9e.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   .github/workflows/reusable-ubuntu-ci.yml

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[JesusPoderoso]

LGTM with green CI