Fix board config modal canceling

What does this MR do?

Fixes canceling the board edit modal.

As far as I can tell, canceling the Edit board / Create board / View scope modal is broken because the modal was given a @toggle handler instead of @cancel handler.

I suppose this was caused by a mixup with using the right sidebar. The right sidebar has a @toggle, but modals only have @cancel.

This MR replaces the modal handler with @cancel to enable the X and Cancel buttons.

Are there points in the code the reviewer needs to double check?

Check that the common modal component really doesn't use @toggle.

Why was this MR needed?

https://gitlab.com/gitlab-org/gitlab-ce/issues/42011 https://gitlab.com/gitlab-org/gitlab-ce/issues/42012

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/42011 https://gitlab.com/gitlab-org/gitlab-ce/issues/42012