[23706] Update fallback branches to use environment variables (backport #6039)

[mergify]

Description

Improve management of related versions by ensuring that fallback_branch: is always assigned from either inputs or environment.

@Mergifyio backport 3.3.x 3.2.x 2.14.x

Note: Remember to update related branches in the backports

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: Any new configuration API has an equivalent XML API (with the corresponding XSD extension)

• Changes are backport compatible: they do NOT break ABI nor change library core behavior.

• 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.

• 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.
• N/A If this is a critical bug fix, backports to the critical-only supported branches have been requested.
• N/A Check CI results: changes do not issue any warning.
• N/A Check CI results: failing tests are unrelated with the changes.

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This is an automatic backport of pull request #6039 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 50ff229 has failed:

On branch mergify/bp/3.3.x/pr-6039
Your branch is up to date with 'origin/3.3.x'.

You are currently cherry-picking commit 50ff2296.
  (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