Description

Based on this comment #47480 (comment) on a small PR
Friendly ping @stof @fabpot as originals reviewers

Questions:

• should it be done massively on the whole codebase to avoid conflicts? or per components?
• can it be automatise in some way?
• can it be prevented to not have new code without on v6 branches? new code or ported code from v4/v5?

Thank you

Example

Symfony v4/5/6

     /**
      * @return string
      */
     public function get()
     {}

Symfony v7

     public function get(): string
     {}