minor #43041 [RateLimiter][Runtime] Remove experimental from README.md (Jean-Beru)

This PR was merged into the 5.4 branch.

Discussion
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[RateLimiter][Runtime] Remove experimental from README.md

| Q             | A
| ------------- | ---
| Branch?       | 5.4
| Bug fix?      | no
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tickets       |  <!-- prefix each issue number with "Fix #", no need to create an issue if none exist, explain below instead -->
| License       | MIT
| Doc PR        | <!-- required for new features -->

Since symfony/symfony#42957 is merged, experimental mentions in RateLimiter and Runtime README.md files can be removed.

Commits
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012d3286d2 [RateLimiter][Runtime] Remove experimental from README.md

Author: fabpot

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
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