Pin middleman-sprockets to 4.0.x.

With middleman-sprockets 4.1, tests fail with:

```
/Users/sengi/src/alphagov/tech-docs-gem/vendor/bundle/ruby/3.3.0/gems/middleman-sprockets-4.1.1/lib/middleman-sprockets/resource.rb:37:in
`block in sprockets_asset': ../../lib/source/stylesheets/manifest.css
(Sprockets::FileNotFound)
```

Author: sengi

govuk_tech_docs.gemspec

@@ -43,7 +43,7 @@ Gem::Specification.new do |spec|
   spec.add_dependency "middleman-compass", "~> 4.0"
   spec.add_dependency "middleman-livereload"
   spec.add_dependency "middleman-search-gds"
-  spec.add_dependency "middleman-sprockets", "~> 4.1"
+  spec.add_dependency "middleman-sprockets", "~> 4.0.0"
   spec.add_dependency "middleman-syntax", "~> 3.4"
   spec.add_dependency "nokogiri"
   spec.add_dependency "openapi3_parser", "~> 0.9.0"