Make ActionController::AllowBrowser::BrowserBlocker private

Despite the fact that the `BrowserBlocker` class is defined after a
`private` modifier, it is still a public constant and is included in the
[documentation][].

To reduce the API surface area of the `allow_browser` feature, this
commit marks the class with a `# :nodoc:` comment.

[documentation]: https://edgeapi.rubyonrails.org/classes/ActionController/AllowBrowser/BrowserBlocker.html

Author: seanpdoyle

actionpack/CHANGELOG.md

@@ -1,5 +1,9 @@
 ## Rails 8.0.0.beta1 (September 26, 2024) ##
 
+*   Make `ActionController::AllowBrowser::BrowserBlocker` private
+
+    *Sean Doyle*
+
 *   Fix non-GET requests not updating cookies in `ActionController::TestCase`.
 
     *Jon Moss*, *Hartley McGuire*

actionpack/lib/action_controller/metal/allow_browser.rb

@@ -60,7 +60,7 @@ def allow_browser(versions:, block:)
         end
       end
 
-      class BrowserBlocker
+      class BrowserBlocker # :nodoc:
         SETS = {
           modern: { safari: 17.2, chrome: 120, firefox: 121, opera: 106, ie: false }
         }