Mark Static as retired in README

Author: CodeSonia

README.md

@@ -1,3 +1,6 @@
+> [!WARNING]  
+> Static has been retired and is no longer in use.
+
 # Static
 
 This application defines global templates for [GOV.UK](https://www.gov.uk) pages. It is used in conjunction with [Slimmer](https://github.com/alphagov/slimmer), which is Rack middleware that takes a HTML response from a Rails app and combines it with a template from Static.