Fix typo in comment and phpstan

[phil-davis]

In PHP 8.1, ArrayAccess has return types specified for methods like offsetGet and so if a class extends ArrayAccess then it now needs to specify the return type (in this case mixed).

But the mixed return type is only available form PHP 8.0 onward. So we can't write that because we still support PHP 7.2 7.3 7.4.

So we have to add https://php.watch/versions/8.1/internal-method-return-types#ReturnTypeWillChange and that suppresses the deprecation notice that PHP 8.1 emits. After PHP 7 is dropped (in the future) then we need to explicitly mention the return type.

[phil-davis]

https://github.com/sabre-io/Baikal/actions/runs/3211896895/jobs/5250379047

Error: Return type mixed of method Flake\Core\Datastructure\ChainLink::offsetGet() is not covariant with tentative return type mixed of method ArrayAccess::offsetGet().
Error: Return type mixed of method Flake\Core\Datastructure\ChainLink::current() is not covariant with tentative return type mixed of method Iterator::current().
Error: Return type mixed of method Flake\Core\Datastructure\ChainLink::key() is not covariant with tentative return type mixed of method Iterator::key().
 ------ ----------------------------------------------------------------------- 
  Line   Core/Frameworks/Flake/Core/Datastructure/ChainLink.php                 
 ------ ----------------------------------------------------------------------- 
  63     Return type mixed of method                                            
         Flake\Core\Datastructure\ChainLink::offsetGet() is not covariant with  
         tentative return type mixed of method ArrayAccess::offsetGet().        
         💡 Make it covariant, or use the #[\ReturnTypeWillChange] attribute    
         to temporarily suppress the error.                                     
  77     Return type mixed of method                                            
         Flake\Core\Datastructure\ChainLink::current() is not covariant with    
         tentative return type mixed of method Iterator::current().             
         💡 Make it covariant, or use the #[\ReturnTypeWillChange] attribute    
         to temporarily suppress the error.                                     
  81     Return type mixed of method Flake\Core\Datastructure\ChainLink::key()  
         is not covariant with tentative return type mixed of method            
         Iterator::key().                                                       
         💡 Make it covariant, or use the #[\ReturnTypeWillChange] attribute    
         to temporarily suppress the error.                                     
 ------ ----------------------------------------------------------------------- 

Error:  [ERROR] Found 3 errors                                                         

I will have at look - these are new complaints by phpstan when running PHP 8.1. I get them locally when I switch to PHP 8.1.

[ByteHamster]

Thanks!