gitlab: renew merge request template

.gitlab/merge_request_templates/default.md

 
 ---
 - Close #...
-- Staging: https://docs.binary.picodata.io/picodata/branch-%{source_branch}/
+- Target branch: %{target_branch}
+- Changes should be cherry-picked to 24.1: yes / no
+- Staging: https://docs.binary.picodata.io/picodata/branch-.../
+<!--
+- Follow-up for !...
+- See also ...
+-->