Refactor code to meet modern coding requirements

[gregor-j]

Even at level 2 PHPStan returns 8 errors:

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  Line   src/JsonRPC/Request/RequestBuilder.php                                             
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  48     Unsafe usage of new static().                                                      
         💡 See: https://phpstan.org/blog/solving-phpstan-error-unsafe-usage-of-new-static  
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  Line   src/JsonRPC/Request/RequestParser.php                                              
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  59     Unsafe usage of new static().                                                      
         💡 See: https://phpstan.org/blog/solving-phpstan-error-unsafe-usage-of-new-static  
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  Line   src/JsonRPC/Response/ResponseBuilder.php                                           
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  89     Unsafe usage of new static().                                                      
         💡 See: https://phpstan.org/blog/solving-phpstan-error-unsafe-usage-of-new-static  
 ------ ----------------------------------------------------------------------------------- 

 ------ ----------------------------------------------------------------------------------- 
  Line   src/JsonRPC/Response/ResponseParser.php                                            
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  42     Unsafe usage of new static().                                                      
         💡 See: https://phpstan.org/blog/solving-phpstan-error-unsafe-usage-of-new-static  
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  Line   src/JsonRPC/Validator/HostValidator.php                          
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  71     Binary operation "-" between 32 and string results in an error.  
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  Line   tests/Validator/HostValidatorTest.php                                                             
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  12     Static method JsonRPC\Validator\HostValidator::validate() invoked with 3 parameters, 2 required.  
  17     Static method JsonRPC\Validator\HostValidator::validate() invoked with 3 parameters, 2 required.  
  31     Static method JsonRPC\Validator\HostValidator::validate() invoked with 3 parameters, 2 required.  
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Serious code changes will be required to fix these issues.