Refs #23269: Remove EOL versions from pr template

Signed-off-by: Mario Dominguez <mariodominguez@eprosima.com>

Author: Mario-DL

.github/pull_request_template.md

@@ -17,7 +17,7 @@
     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 2.1.x 1.4.x 1.2.x 1.0.x -->
+<!-- @Mergifyio backport 1.4.x -->
 
 <!-- If an issue is already opened, please uncomment next line with the corresponding issue number. -->
 <!-- Fixes #(issue) -->