Merge branch '5.4' into 6.0

* 5.4:
  Silence Psalm errors about Enum classes
  [RateLimiter][Runtime] Remove experimental from README.md
  [Mailer] Adding support for TagHeader and MetadataHeader to the Sendgrid API transport

Author: derrabus

README.md

@@ -3,11 +3,6 @@ Runtime Component
 
 Symfony Runtime enables decoupling applications from global state.
 
-**This Component is experimental**.
-[Experimental features](https://symfony.com/doc/current/contributing/code/experimental.html)
-are not covered by Symfony's
-[Backward Compatibility Promise](https://symfony.com/doc/current/contributing/code/bc.html).
-
 Resources
 ---------