Fix two remaining PHPStan test failures [#3556885]

Problem/Motivation

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  Line   modules/purview_unified_catalog_ui/src/Controller/UnifiedTermMetadata  
         Controller.php                                                         
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  207    Call to deprecated method renderPlain() of interface                   
         Drupal\Core\Render\RendererInterface:                                  
         in drupal:10.3.0 and is removed from drupal:12.0.0. Use                
           \Drupal\Core\Render\RendererInterface::renderInIsolation() instead.  
         🪪  method.deprecated                                                  
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  Line   tests/src/Unit/PurviewGovernanceDomainApiClientTest.php                
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  71     Call to deprecated method isType() of class PHPUnit\Framework\Assert:  
         https://github.com/sebastianbergmann/phpunit/issues/6052               
         🪪  staticMethod.deprecated                                            
  71     Call to deprecated method isType() of class PHPUnit\Framework\Assert:  
         https://github.com/sebastianbergmann/phpunit/issues/6052               
         🪪  staticMethod.deprecated                                            
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