Fix supported branches in PR template backports section

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.

[x] 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.

[x] The PR has a milestone assigned.
[x] The title and description correctly express the PR's purpose.
[x] 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.