Convert ViewsRelationship plugin annotation to PHP 8 attribute for Drupal 11 compatibility

Problem/Motivation

As per the change record Plugin implementations should use PHP attributes instead of annotations, Drupal 11 requires plugins to use PHP 8 attributes instead of
doctrine annotations.

The FlagViewsRelationship plugin in src/Plugin/views/relationship/FlagViewsRelationship.php still uses the old annotation format:

/**                                                                                                                                                                                                            
   * Provides a views relationship to select flag content by a flag.                                                                                                                                                        
   *                                                                                                                                                                                                                        
   * @ViewsRelationship("flag_relationship")                                                                                                                                                                                
   */                                                                                                                                                                                                                       
  class FlagViewsRelationship extends RelationshipPluginBase implements CacheableDependencyInterface {                                                                                                                      
  

This causes the relationship to be marked as "Broken" in Views when running on Drupal 11.3.x, resulting in errors like:

• Warning: Undefined array key "base" in Drupal\views\Plugin\views\query\QueryPluginBase->getEntityTableInfo()
• InvalidArgumentException: A valid cache entry key is required

Steps to reproduce

1. Install Flag 5.0.3 on Drupal 11.x
2. Create a view with a Flag relationship
3. The relationship appears as "Broken/missing handler"

Proposed resolution

Convert the annotation to PHP 8 attribute format:

use Drupal\views\Attribute\ViewsRelationship;                                                                                                                                                                  
                                                                                                                                                                                                                            
  /**                                                                                                                                                                                                                       
   * Provides a views relationship to select flag content by a flag.                                                                                                                                                        
   */                                                                                                                                                                                                                       
  #[ViewsRelationship("flag_relationship")]                                                                                                                                                                                 
  class FlagViewsRelationship extends RelationshipPluginBase implements CacheableDependencyInterface {                                                                                                                      
  

Remaining tasks

• Review if other plugins in the Flag module also need conversion to attributes
• Create patch
• Add test coverage

User interface changes

None

API changes

None

Data model changes

None