Use git-pull-request icon for admin menu entry

op-logo-gitea does not exist in the OpenProject octicons set.
Use the standard git-pull-request octicon as a placeholder,
matching what is available.

lib/open_project/gitea_integration/engine.rb

@@ -58,7 +58,7 @@ module OpenProject::GiteaIntegration
                   parent: :admin_integrations,
                   if: ->(_) { User.current.admin? },
                   caption: "Gitea",
-                  icon: :"op-logo-gitea"
+                  icon: "git-pull-request"
       end
 
       project_module(:gitea, dependencies: :work_package_tracking) do