PHP 8.1 support

- This is currently blocked by Fatfree https://github.com/bcosca/fatfree/issues/1241, should be fixed by https://github.com/bcosca/fatfree-core/pull/332 ~~With https://github.com/fossar/selfoss/pull/1296, we will no longer use F3.~~
- https://github.com/violet-php/streaming-json-encoder/pull/4