Adding 2.3.x to the backported versions in PR template

Signed-off-by: Emilio Cuesta <emiliocuesta@eprosima.com>

Author: emiliocuestaf

.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 2.2.x 1.4.x -->
+<!-- @Mergifyio backport 2.3.x 2.2.x 1.4.x -->
 
 <!-- If an issue is already opened, please uncomment next line with the corresponding issue number. -->
 <!-- Fixes #(issue) -->