fix(rule): Fix phpstan fail for result of array_column

[BitScout]

Fixes the build error with the message

------ ----------------------------------------------------------------------- 
  Line   src/Rules/BannedNodesRule.php                                          
 ------ ----------------------------------------------------------------------- 
  38     Property Ekino\PHPStanBannedCode\Rules\BannedNodesRule::$bannedNodes   
         (array<array<string, array<string>>>) does not accept                  
         array<array<string>|int|string, array<string, array<string>|string>>.  
 ------ -----------------------------------------------------------------------

See https://github.com/ekino/phpstan-banned-code/runs/5225983073?check_suite_focus=true

This might be linked to this change: phpstan/phpstan-src#948