Drop PHP 8.1

[eucyt]

PHP 8.1 has reached its EOL. Support for it in this repository will also be dropped.

https://www.php.net/supported-versions.php

[habara-k]

git grep -n -r "8\.1"
.github/scripts/check-eol-newrelease.cjs:5: * @property {string|number} cycle - Release cycle (e.g. "1.20", 8.1, etc.)
.github/workflows/generate-code.yml:56:          wget https://github.com/phpDocumentor/phpDocumentor/releases/download/v3.8.1/phpDocumentor.phar
CONTRIBUTING.md:74:$ wget https://github.com/phpDocumentor/phpDocumentor/releases/download/v3.8.1/phpDocumentor.phar
README.md:167:$ wget https://github.com/phpDocumentor/phpDocumentor/releases/download/v3.8.1/phpDocumentor.phar

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