fix: add ondrej/php PPA when PHP version not in default repos

The install script was failing on Ubuntu 22.04 because PHP 8.2 is not
available in default repositories. This fix checks if the requested
PHP version is available, and if not, adds the ondrej/php PPA which
provides all supported PHP versions.
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f