Refs #24136. Update PR template.

Signed-off-by: Miguel Company <miguelcompany@eprosima.com>

Author: MiguelCompany

.github/pull_request_template.md

@@ -19,7 +19,7 @@
     In case of bug fixes, please provide the list of supported branches where this fix should be also merged.
     Please uncomment following line, adjusting the corresponding target branches for the backport.
 -->
-<!-- @Mergifyio backport 3.4.x 3.2.x 2.14.x -->
+<!-- @Mergifyio backport 3.2.x 2.14.x -->
 
 <!-- If an issue is already opened, please uncomment next line with the corresponding issue number. -->
 <!-- Fixes #(issue) -->