$ bin/rails test test/functional/admin_controller_test.rb
.
.
.
DEPRECATION WARNING: render file: should be given the absolute path to a file (called from _app_views_layouts_admin_html_erb___682843397516142584_70111060943800 at /path/to/redmine/app/views/layouts/admin.html.erb:8)

The behavior has changed by this commit:
https://github.com/rails/rails/commit/c7820d8124c854760a4d288334f185de2fb99446

According to the commit log, the only difference between @render file:@ and @render template:@ before Rails 6 is that @render file:@ can specify the absolute path of templates.

@app/views/layouts/admin.html.erb@ does not use an absolute path to specify the template, so you can suppress deprecation warning by replacing @render file:@ with @render template:@.

diff --git a/app/views/layouts/admin.html.erb b/app/views/layouts/admin.html.erb

index 8f23129b4..0ce74d104 100644

--- a/app/views/layouts/admin.html.erb

+++ b/app/views/layouts/admin.html.erb

@@ -5,4 +5,4 @@

   <% end %>

 <% end %>


-<%= render :file => "layouts/base" %>

+<%= render :template => 'layouts/base' %>