Bump lowest maintained version to 6.4

Author: phansys

README.md

@@ -27,8 +27,8 @@ We love contributors! For more information on how you can contribute, please rea
 the [Symfony Docs Contributing Guide](https://symfony.com/doc/current/contributing/documentation/overview.html).
 
 > [!IMPORTANT]
-> Use `5.4` branch as the base of your pull requests, unless you are documenting a
-> feature that was introduced *after* Symfony 5.4 (e.g. in Symfony 7.1).
+> Use `6.4` branch as the base of your pull requests, unless you are documenting a
+> feature that was introduced *after* Symfony 6.4 (e.g. in Symfony 7.2).
 
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