Update to Symfony 8.1

Ran into https://github.com/sebastianbergmann/phpunit/issues/6545 so we need to bump PHPUnit as well, though still getting errors locally on unit tests.

https://github.com/symfony/symfony/releases/tag/v8.1.0-BETA3 is out.

Rebased and updated to Symfony 8.1.1

$ composer-lock-diff --from=main --no-links
+------------------------------------+---------+---------+
| Production Changes                 | From    | To      |
+------------------------------------+---------+---------+
| symfony/console                    | v8.0.11 | v8.1.1  |
| symfony/dependency-injection       | v8.0.10 | v8.1.1  |
| symfony/deprecation-contracts      | v3.7.0  | v3.7.1  |
| symfony/error-handler              | v8.0.8  | v8.1.0  |
| symfony/event-dispatcher           | v8.0.9  | v8.1.1  |
| symfony/event-dispatcher-contracts | v3.7.0  | v3.7.1  |
| symfony/filesystem                 | v8.0.11 | v8.1.0  |
| symfony/finder                     | v8.0.8  | v8.1.1  |
| symfony/http-foundation            | v8.0.13 | v8.1.1  |
| symfony/http-kernel                | v8.0.12 | v8.1.1  |
| symfony/mailer                     | v8.0.12 | v8.1.1  |
| symfony/mime                       | v8.0.12 | v8.1.0  |
| symfony/polyfill-intl-grapheme     | v1.37.0 | v1.38.1 |
| symfony/polyfill-intl-normalizer   | v1.37.0 | v1.38.0 |
| symfony/polyfill-mbstring          | v1.37.0 | v1.38.2 |
| symfony/polyfill-php86             | v1.37.0 | v1.38.0 |
| symfony/process                    | v8.0.11 | v8.1.0  |
| symfony/psr-http-message-bridge    | v8.0.8  | v8.1.0  |
| symfony/routing                    | v8.0.13 | v8.1.0  |
| symfony/runtime                    | v8.0.12 | v8.1.0  |
| symfony/serializer                 | v8.0.10 | v8.1.1  |
| symfony/service-contracts          | v3.7.0  | v3.7.1  |
| symfony/string                     | v8.0.11 | v8.1.0  |
| symfony/translation-contracts      | v3.7.0  | v3.7.1  |
| symfony/validator                  | v8.0.10 | v8.1.1  |
| symfony/var-dumper                 | v8.0.8  | v8.1.1  |
| symfony/var-exporter               | v8.0.9  | v8.1.1  |
| symfony/yaml                       | v8.0.12 | v8.1.1  |
| symfony/polyfill-deepclone         | NEW     | v1.40.0 |
+------------------------------------+---------+---------+

+---------------------------+---------+---------+
| Dev Changes               | From    | To      |
+---------------------------+---------+---------+
| phpunit/php-code-coverage | 12.5.3  | 12.5.6  |
| phpunit/phpunit           | 12.5.14 | 12.5.24 |
| sebastian/comparator      | 7.1.4   | 7.1.6   |
| sebastian/environment     | 8.0.4   | 8.1.0   |
| symfony/browser-kit       | v8.0.8  | v8.1.1  |
| symfony/css-selector      | v8.0.9  | v8.1.0  |
| symfony/dom-crawler       | v8.0.12 | v8.1.1  |
| symfony/lock              | v8.0.9  | v8.1.1  |
+---------------------------+---------+---------+

Crediting @mstrelan for #3607060: ConsoleCompilerPass incompatible with symfony/console ≥ 8.1 which I merged in here, as all the test fails look related to that.

Bumping to critical in the hope I can avoid rerolling this yet again :D

Rebased and fixed or responded to all comments.

I don't think it's intended, but it's a side effect of Symfony now using get/set property hooks in Request/Response. It's related to https://github.com/sebastianbergmann/phpunit/issues/6545 from what I remember which is why we had to upgrade PHPUnit here. There might be a better way, not sure?

To be honest though it makes the code cleaner where we can actually use real Request/Response objects instead of creating mocks.

The comment "The thing is: those public properties were not meant to be set from outside" also I guess means that we were never meant to do this anyway, even in tests.