[v6] - PHP 8.1

[DerManoMann]

Fixes #1669

• Update general docs
• Flesh out migration guide
• Refactor Util helper
• Remove other deprecated elements (if any)
• Remove Generator::scan() and update Generator reference
• Add phpdoc parser / type handling refactor (adding to v5 first)
• Remove @requires PHP 8.1 test annotations
• Remove code checks for 8.1 features
• Remove AbstractAnnotation __set() and __get() methods
• Bump phpunit deps to ^10.0

[Kiina]

So is the type-info-extras not landing in v6? Since it requires 8.2 while the min version is bumped to 8.1.

[DerManoMann]

So is the type-info-extras not landing in v6? Since it requires 8.2 while the min version is bumped to 8.1.

The type-info code itself is ready, including a rough polyfill. Hopefully it will be more consistent than what we have now and if type-info-extras is detected it will use that instead.

I thought about a PR to just add that so it can be reviewed - but not used yet; maybe worth doing.