Move up to phpstan major version 2

[phil-davis]

phpstan 2.1.1 reports:

$ composer phpstan
> phpstan analyse lib tests
Note: Using configuration file /home/phil/git/sabre-io/http/phpstan.neon.
 40/40 [▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓] 100%

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  Line   lib/Auth/Digest.php                                                                                              
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         Ignored error pattern #^Else branch is unreachable because ternary operator condition is always true.$# in path  
         /home/phil/git/sabre-io/http/lib/Auth/Digest.php was not matched in reported errors.                             
 ------ ----------------------------------------------------------------------------------------------------------------- 

 ------ -------------------------------------------------------------------------------------- 
  Line   lib/Message.php                                                                       
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  88     PHPDoc tag @var with type int|string|null is not subtype of native type string|null.  
         🪪  varTag.nativeType                                                                  
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 ------ --------------------------------------------- 
  Line   lib/Sapi.php                                 
 ------ --------------------------------------------- 
  89     Loose comparison via "==" is not allowed.    
         🪪  equal.notAllowed                          
         💡 Use strict comparison via "===" instead.   
 ------ --------------------------------------------- 

 ------ --------------------------------------------- 
  Line   tests/HTTP/Auth/AWSTest.php                  
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  203    Loose comparison via "==" is not allowed.    
         🪪  equal.notAllowed                          
         💡 Use strict comparison via "===" instead.   
 ------ --------------------------------------------- 

 ------ --------------------------------------------- 
  Line   tests/HTTP/Auth/DigestTest.php               
 ------ --------------------------------------------- 
  169    Loose comparison via "==" is not allowed.    
         🪪  equal.notAllowed                          
         💡 Use strict comparison via "===" instead.   
 ------ --------------------------------------------- 

 ------ --------------------------------------------- 
  Line   tests/HTTP/ClientTest.php                    
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  540    No error to ignore is reported on line 540.  
  560    No error to ignore is reported on line 560.  
  580    No error to ignore is reported on line 580.  
 ------ --------------------------------------------- 

 ------ ----------------------------------------------------------------------------------------------------- 
  Line   tests/HTTP/RequestTest.php                                                                           
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  40     Annotation "@backupGlobals" is invalid, "@backupGlobals" should be followed by a space and a value.  
         🪪  phpunit.invalidPhpDoc                                                                             
 ------ ----------------------------------------------------------------------------------------------------- 

                                                                                                                        
 [ERROR] Found 9 errors                                                                                                 
                                                                                                                        

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