Allow PHP 8.3

[L3o-pold]

Description

Allow PHP 8.3 and drop PHP 8.1

Wait for https://github.com/kreait/firebase-php/pull/847

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have made corresponding changes to the documentation

[ankurk91]

There is no need to drop support for php 8.1. @jeromegamez

[jeromegamez]

Merged with 7cd191ac9a3cf229f4c9c6073c69c3e2a536c6e7, thank you @L3o-pold!