Bump phpstan to major version 2

[phil-davis]

It reports:

$ composer phpstan
> phpstan analyse
Note: Using configuration file /home/phil/git/sabre-io/xml/phpstan.neon.
 39/39 [▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓] 100%

 ------ ----------------------------------------------------------------------------- 
  Line   lib/Serializer/functions.php                                                 
 ------ ----------------------------------------------------------------------------- 
  184    Call to function is_string() with string will always evaluate to true.       
         🪪  function.alreadyNarrowedType                                              
  202    Call to function is_object() with object will always evaluate to true.       
         🪪  function.alreadyNarrowedType                                              
         💡 Remove remaining cases below this one and this error will disappear too.   
  204    Call to function is_null() with *NEVER* will always evaluate to true.        
         🪪  function.alreadyNarrowedType                                              
 ------ ----------------------------------------------------------------------------- 

 ------ ----------------------------------------------------------------------------------------------------------------------- 
  Line   tests/Sabre/Xml/ReaderTest.php                                                                                         
 ------ ----------------------------------------------------------------------------------------------------------------------- 
  342    Call to static method PHPUnit\Framework\Assert::assertIsArray() with array<LibXMLError> will always evaluate to true.  
         🪪  staticMethod.alreadyNarrowedType                                                                                    
 ------ ----------------------------------------------------------------------------------------------------------------------- 

 ------ --------------------------------------------------------------------------------------------------------------------------------- 
  Line   tests/Sabre/Xml/ServiceTest.php                                                                                                  
 ------ --------------------------------------------------------------------------------------------------------------------------------- 
  189    Call to static method PHPUnit\Framework\Assert::assertIsObject() with Sabre\Xml\PropFindTestAsset will always evaluate to true.  
         🪪  staticMethod.alreadyNarrowedType                                                                                              
         💡 Because the type is coming from a PHPDoc, you can turn off this check by setting treatPhpDocTypesAsCertain: false in your      
            phpstan.neon.                                                                                                                 
 ------ --------------------------------------------------------------------------------------------------------------------------------- 

                                                                                                                        
 [ERROR] Found 5 errors                                                                                                 
                                                                                                                        

Script phpstan analyse handling the phpstan event returned with error code 1

ToDo: sort out the above so it passes.