feat: change codegen to use php 8.1

[ctran88]

What's New?

generate.sh

• changed to use php-nextgen instead of php to make the generated code use PHP 8.1
• only copies the source code into generated/lib to "whitelist" instead of removing unnecessary meta files to "blacklist" them
  • this also addresses several new files that were not generated before, but are still not needed now
• updates generator script to codemod generated API definitions so that the return values are void instead of a union of error models
  • for some reason php-nextgen is generating return types like OpenApi\Client\Model\Model401Error|OpenApi\Client\Model\Model404Error|etc... instead of void like php used to, so this was necessary to correct the type errors from those operations

custom/

• changes hand-written models to use enums instead of strings

generated/

• deleted meta or ci files that are not necessary
• ran generate.sh openapi.json against the current published spec. most of the changes seem to be
  • use Class instead of using the global import syntax \Class
  • adding types to variables, params, and function return signatures
  • adding a type unioned |null where it was previously implicitly typed as such

test with

BRANCH=PSG-5559-use-enums task php:test-release

Screenshots (if appropriate):

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have manually tested my code thoroughly
• I have added/updated inline documentation for public facing interfaces if relevant
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing integration and unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

Additional context

[ctran88]

removed unnecessary readonly access modifiers to allow devs to update the arg objects after instantiation

[ctran88]

this directory and the one below that is also removed no longer exist