Merge branch '6.4' into 7.0

* 6.4:
  [Translation] Improve tests coverage
  [Routing] Add redirection.io as sponsor of versions 6.4/7.0/7.1
  don't check parameter values if they are not set
  [HttpClient] Add Innovative Web AG (i-web) as sponsor of version 6.4/7.0
  [Serializer] Fix normalization relying on allowed attributes only
  Minor @var doc update
  [Translation] Remove `@internal` from abstract testcases
  [VarExporter] Work around php/php-src#12695 for lazy objects, fixing nullsafe-related behavior
  [Validator] Add missing translations for Bulgarian #51931
  [VarExporter] Fix serializing objects that implement __sleep() and that are made lazy
  fix typo
  Document BC break with $secret parameter introduction
  Bump Symfony version to 6.4.0
  Update VERSION for 6.4.0-RC2
  Update CHANGELOG for 6.4.0-RC2
  [String] Fix Inflector for 'icon'
  [Validator] Made tests forward-compatible with ICU 72.1

Author: nicolas-grekas

README.md

@@ -41,6 +41,17 @@ $url = $generator->generate('blog_show', [
 // $url = '/blog/my-blog-post'
 ```
 
+Sponsor
+-------
+
+The Routing component for Symfony 7.0 is [backed][1] by [redirection.io][2].
+
+redirection.io logs all your website’s HTTP traffic, and lets you fix errors
+with redirect rules in seconds. Give your marketing, SEO and IT teams the
+right tool to manage your website traffic efficiently!
+
+Help Symfony by [sponsoring][3] its development!
+
 Resources
 ---------
 
@@ -49,3 +60,7 @@ Resources
  * [Report issues](https://github.com/symfony/symfony/issues) and
    [send Pull Requests](https://github.com/symfony/symfony/pulls)
    in the [main Symfony repository](https://github.com/symfony/symfony)
+
+[1]: https://symfony.com/backers
+[2]: https://redirection.io
+[3]: https://symfony.com/sponsor