[17351] Remove 2.7.x branch from Mergifyio suggestions

[jsan-rt]

Remove 2.7.x branch from Mergifyio suggestions on the PR template

Description

Contributor Checklist

• Commit messages follow the project guidelines.
• The code follows the style guidelines of this project.
• [NA] 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
• [NA] Any new/modified methods have been properly documented using Doxygen.
• Changes are ABI compatible.
• Changes are API compatible.
• [NA] New feature has been added to the versions.md file (if applicable).
• [NA] New feature has been documented/Current behavior is correctly described in the documentation.
• [NA] Applicable backports have been included in the description.

Reviewer Checklist

• The PR has a milestone assigned.
• Check contributor checklist is correct.
• Check CI results: changes do not issue any warning.
• Check CI results: failing tests are unrelated with the changes.

[EduPonz]

We do not need to run CI for these kinds of PRs. Could have used the skip-ci label