docs: remove v2 warning

@zkochan If you have tested that it work with newer Node.js versions, this warning can be removed.

README.md

@@ -1,7 +1,3 @@
-> ## :warning: Upgrade from v2!
->
-> The v2 version of this action [has stopped working](https://github.com/pnpm/action-setup/issues/135) with newer Node.js versions. Please, upgrade to the latest version to fix any issues.
-
 # Setup pnpm
 
 Install pnpm package manager.