Adding an issues template for backporting code

Signed-off-by: Benoit Mortier <benoit.mortier@opensides.be>

.gitlab/issue_templates/Backporting.md

+### Description
+
+<!-- Required -->
+<!-- Description of the issue -->
+
+### Source FusionDirectory Version
+
+<!-- Required -->
+<!-- FusionDirectory version where the code come from -->
+
+### Destination FusionDirectory Version
+
+<!-- Required -->
+<!-- FusionDirectory version where the code sould go -->
+
+### Plugin with the new code
+
+<!-- Required -->
+<!-- Name of the plugin -->
+
+### Reason of Backporting code
+
+<!-- Required -->
+
+### What defect does it correct
+
+<!-- Required -->
+<!-- Why do we backport this code -->
+
+### Additional Information
+<!-- optional -->
+<!-- Any additional information, configuration or data that might be necessary to reproduce the issue. -->