Fix direct access to individual design on deprecated issue route

What does this MR do?

This MR fixes a bug when directly accessing an individual Design Management design on the deprecated issue route.

71d92460 introduced a change to issue routes, deprecating the old ones, but still supporting them. There was added a special variable in issues/_discussion.html.haml to inform the Vue app about which of the two possible routes the Vue app was mounted on.

The calculation of deprecated_route was previously only returning true if the user was looking at:

• issues/1
• issues/1/designs

But not:

• issues/1/designs/mydesign.png

In Rails, everything after the designs segment of the route is wildcarded as a single param[:vueroute] so it can be handled by Vue.

An example of an affected route is:

https://gitlab.com/gitlab-org/gitlab/issues/22702/designs/Wiki%20editing.png

which at time of writing does not work. Whereas the non-deprecated route:

https://gitlab.com/gitlab-org/gitlab/-/issues/22702/designs/Wiki%20editing.png

does.

• Issue: #211833 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Closes #211833 (closed)