[php-symfony] Symfony6 support (#11810)

* support symfony6

* fix issues with StrictJsonDeserialization

* regenerate samples

* add suggestions

* update samples

* support php 7.4 and symfony 5

* allow versions based on semantic versioning

* regenerate sample

* change method of determining result types

* update samples

* describe usage of bundle in symfony app

* better documentation

* fix duplicate auth methods

* do not set namespace for default types

* fix UploadedFile type

* next try fixing auth

* regenerate samples

* fix: auth method shall not be duplicated

* Revert "fix duplicate auth methods"

This reverts commit 0dc418737b.

* chore: regenerate samples

* fix tests

* regenerate sample

* more fixes for tests

* update tests

* add kernel shutdown

Co-authored-by: Benjamin Haeublein <benjaminh@testing-vm.lan.benjaminh.de>
Co-authored-by: Renaud de Chivré <renaud@tahitiwebdesign.com>

samples/server/petstore/php-symfony/SymfonyBundle-php/.travis.yml

@@ -1,8 +1,7 @@
 language: php
 dist: trusty
 php:
-    - 7.1.3
-    - 7.2
+    - 8.1.1
 
 install:
   - composer install --dev --no-interaction