Rename modal.vue to deprecated_modal.vue

What does this MR do?

Rename old modal Vue component to make it clear that we have a new one.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Review
  • Has been reviewed by Frontend

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/42170, https://gitlab.com/gitlab-org/gitlab-ce/issues/28849