[Symfony 6] Add "static" type hint to the methods overriding Symfony\Component\HttpFoundation\Response::sendContent()

Problem/Motivation

Method "Symfony\Component\HttpFoundation\Response::sendContent()" will return "static" as of its next major version. Doing the same in child class "Drupal\big_pipe\Render\BigPipeResponse" will be required when upgrading.

Steps to reproduce

Proposed resolution

Add the "static" return type hint.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet