Rename default MR template

Use default.md instead of default-template.md in the MR default template file as the instructions say:

In GitLab 14.8 and later, create a merge request template named Default.md (case insensitive) and save it in .gitlab/merge_request_templates/.

More info in https://docs.gitlab.com/ee/user/project/description_templates.html#set-a-default-template-for-merge-requests-and-issues