Resolve PHPStan issues around dependency injection and deprecated methods

Problem/Motivation

The PHPStan pipeline is failing with the following:

------ ---------------------------------------------------------------------- 
  Line   src/CountryPathDomainAliasForm.php                                    
 ------ ---------------------------------------------------------------------- 
  53     \Drupal calls should be avoided in classes, use dependency injection  
         instead                                                               
         🪪  globalDrupalDependencyInjection.useDependencyInjection            
 ------ ---------------------------------------------------------------------- 
 ------ ----------------------------------------------------------------------- 
  Line   src/CountryPathDomainForm.php                                          
 ------ ----------------------------------------------------------------------- 
  31     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                                                  
 ------ ----------------------------------------------------------------------- 
 ------ ---------------------------------------------------------------------- 
  Line   src/CountryPathDomainListBuilder.php                                  
 ------ ---------------------------------------------------------------------- 
  25     \Drupal calls should be avoided in classes, use dependency injection  
         instead                                                               
         🪪  globalDrupalDependencyInjection.useDependencyInjection            
 ------ ---------------------------------------------------------------------- 

Steps to reproduce

Run the gitlab pipelines!!

Proposed resolution

Resolve the issues by replacing the deprecated renderPlain() method and using proper dependency injection.

Remaining tasks

Replace deprecated renderPlain() method in src/CountryPathDomainForm.php.
Use proper dependency injection in src/CountryPathDomainAliasForm.php.
Use proper dependency injection in src/CountryPathDomainListBuilder.php.

User interface changes

N/A

API changes

N/A

Data model changes

N/A