Resolve "Design Management GraphQL Uses Deprecated Field `designs`"

What does this MR do?

Replaces uses of designs field with designCollection within GraphQL queries (of which there is currently only 1), and updates uses of said queries results to use designCollection key.

Note that at this point it seems safe to remove the designs field from the GraphQL schema as it's no longer in use within GitLab. However, it is still used in rspec tests, and for this reason I've left those definitions in for now.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• 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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team