Fix spec failure in spec/features/markdown/markdown_spec.rb

We need to exclude Mermaid from the list of Kroki filters because
Mermaid is implemented natively in GitLab. This was accidentally removed
in https://gitlab.com/gitlab-org/gitlab/-/merge_requests/49304.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/321771

lib/gitlab/kroki.rb

@@ -13,9 +13,8 @@ module Gitlab
         packetdiag
         rackdiag
       ].freeze
-    DIAGRAMS_FORMATS = ::AsciidoctorExtensions::Kroki::SUPPORTED_DIAGRAM_NAMES
-    DIAGRAMS_FORMATS_WO_PLANTUML = DIAGRAMS_FORMATS
-                                        .reject { |diagram_type| diagram_type == 'plantuml' }
+    DIAGRAMS_FORMATS = (::AsciidoctorExtensions::Kroki::SUPPORTED_DIAGRAM_NAMES - %w(mermaid)).freeze
+    DIAGRAMS_FORMATS_WO_PLANTUML = (DIAGRAMS_FORMATS - %w(plantuml)).freeze
 
     # Get the list of diagram formats that are currently enabled
     #