docs: move Bug template to Default to improve discoverability

Description

This MR renames Bug.md template to Default.md so that everyone who creates a new issue has the bug template prefilled.

This will improve discoverability because the vast majority of our bug reports don't adhere to the template

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

Thank you @splattael for this idea 🙇