Use 'General Change.md' as default MR description

Summary

Follow-up to !585 (merged)

Renames 'General Change.md' to 'Default.md' so it is automatically used as the default merge request description.

Related documentation: https://docs.gitlab.com/ee/user/project/description_templates.html#supported-variables-in-merge-request-templates

This ensures that the '%{first_multiline_commit}' variable works as expected, given the note in the documentation:

This feature is available only for the default template.

The default template docs state that the file needs to be named 'Default.md' (case insensitive).