Merge branch 'reserve-robots-txt-namespace' into 'master'

Add `robots.txt` to the list of reserved namespaces

See #22759

See merge request !6789

app/validators/namespace_validator.rb

@@ -24,6 +24,7 @@ class NamespaceValidator < ActiveModel::EachValidator
     projects
     public
     repository
+    robots.txt
     s
     search
     services