When moving an issue we copy the design collection. Copying the design
collection involves adding commits to the design repository. Some users
have permission to move the issue (`:admin_issue` on the target
project), but not permission to push to the design repository of that
project and are being denied by
`GitAccessDesign#check_can_create_design!`.

Instead we now use the `Project#default_owner` to perform all of the git
actions instead of the `current_user`.

We still record the correct `Version#author` of the design in PostgreSQL
and still check the user can `:admin_issue` the target issue before
performing the copy.

https://gitlab.com/gitlab-org/gitlab/-/issues/263102