Fix supported branches in PR template backports section

[JesusPoderoso]

Description

The PR template shows as possible branches to backport the current 2.2.x branch instead of 2.1.x.
This PR fixes it.

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 backport compatible: they do NOT break ABI nor change library core behavior.
• N/A Changes are API compatible.
• N/A New feature has been added to the versions.md file (if applicable).
• 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.
• N/A Check CI results: changes do not issue any warning.
• N/A Check CI results: CI pass and failing tests are unrelated with the changes.