Merge branch 'docs-designs-add-badge' into 'master'

Docs: add missing product tier badge on Designs

See merge request gitlab-org/gitlab!29770

doc/user/project/issues/design_management.md

-# Design Management
+# Design Management **(PREMIUM)**
 
 > [Introduced](https://gitlab.com/groups/gitlab-org/-/epics/660) in [GitLab Premium](https://about.gitlab.com/pricing/) 12.2.