Fix broken pipelines due to PHPSTAN errors and PHPUNIT warnings

Problem/Motivation

Currently, build pipelines are failing for the 2.2.x branch see:
https://git.drupalcode.org/project/image_link_formatter/-/pipelines/899367

Screenshot:

 

Failing jobs : PHPSTAN (all versions) due to errors :

 ------ ----------------------------------------------------------------------- 
  Line   src/Plugin/Field/FieldFormatter/ImageLinkFormatterTrait.php (in        
         context of class                                                       
         Drupal\image_link_formatter\Plugin\Field\FieldFormatter\ImageLinkForm  
         atter)                                                                 
 ------ ----------------------------------------------------------------------- 
  38     Drupal\Core\DependencyInjection\DependencySerializationTrait does not  
         support private properties.                                            
         🪪  dependencySerializationTraitProperty.unsupportedPrivateProperty    
         💡  See https://www.drupal.org/node/3110266                            
  45     Drupal\Core\DependencyInjection\DependencySerializationTrait does not  
         support private properties.                                            
         🪪  dependencySerializationTraitProperty.unsupportedPrivateProperty    
         💡  See https://www.drupal.org/node/3110266                            
 ------ ----------------------------------------------------------------------- 
 ------ ----------------------------------------------------------------------- 
  Line   src/Plugin/Field/FieldFormatter/ImageLinkFormatterTrait.php (in        
         context of class                                                       
         Drupal\responsive_image_link_formatter\Plugin\Field\FieldFormatter\Re  
         sponsiveImageLinkFormatter)                                            
 ------ ----------------------------------------------------------------------- 
  38     Drupal\Core\DependencyInjection\DependencySerializationTrait does not  
         support private properties.                                            
         🪪  dependencySerializationTraitProperty.unsupportedPrivateProperty    
         💡  See https://www.drupal.org/node/3110266                            
  45     Drupal\Core\DependencyInjection\DependencySerializationTrait does not  
         support private properties.                                            
         🪪  dependencySerializationTraitProperty.unsupportedPrivateProperty    
         💡  See https://www.drupal.org/node/3110266                            
 ------ ----------------------------------------------------------------------- 
 [ERROR] Found 4 errors  

For PHPUNIT : the jobs are not failing, but next minor and next major are completing with warnings due to deprecation messages, mostly unrelated to the module itself.

Steps to reproduce

Run build pipeline for branch 2.2.x at:
https://git.drupalcode.org/project/image_link_formatter/-/pipelines/new

Proposed resolution

Fix PHPSTAN reported errors: most likely just change the problematic properties from private to protected.

For PHPUNIT jobs, I think we should probably try ignoring deprecation errors :
Deprecation messages related with the module should be prompted by PHPSTAN, upgrade status and other validation jobs.
There is no real interest for the module's maintenance in keeping the deprecation messages triggered by the inherited Tests classes from Core (ImageFormatter and ResponsiveImageFormatter).

If deprecations are not anticipated, that's not necessarily a big deal either for the module, since the tests/jobs would fail with errors and we would be able to know exactly which deprecated code to replace.

Remaining tasks

User interface changes

API changes

Data model changes