Allow `issue_templates/default.md` to be the default issue template at Core/Free level

Problem to solve

As a GitLab CE user, I want to be able to set a default issue template so I can auto-assign issues to myself and apply a default label.

Intended users

DevOps Engineer, Software Developer (but really, anybody who is creating an issue and doesn't want to have to manually select the standard template from a dropdown list which is not particularly easy to use with only the keyboard).

User experience goal

The user should be able to use .gitlab/issue_templates/default.md as the default issue template on all GitLab editions, with the current UI to select different templates being reserved for paid users.

Proposal

If a .gitlab/issue_templates/default.md exists, use it as the default issue template unless this is changed from the UI.

The UI to allow selecting more meaningfully-named templates should continue to be gated to the current tier

Further details

While #15028 would solve the label part of this problem for me, I am the sole developer on this (free, unpaid) project, so being able to /assign @myself as part of the default template would be very useful.

Permissions and Security

Documentation

Availability & Testing

What does success look like, and how can we measure that?

What is the type of buyer?

The UI for selecting a default template is currently at the Starter/Bronze level. I believe a more basic version, without the UI and requiring a specific filename, would be reasonable at Core/Free, rather than totally denying the ability to have default templates at all.