Inject the token service in NotifyGroupNode

Problem/Motivation

phpstan was complaining about:

 ------ ---------------------------------------------------------------------- 
  Line   src/Plugin/Group/Relation/NotifyGroupNode.php                         
 ------ ---------------------------------------------------------------------- 
  289    \Drupal calls should be avoided in classes, use dependency injection  
         instead                                                               
 ------ ---------------------------------------------------------------------- 

That's for this line:

    return \Drupal::token()->replace($subject_config, $context);

For #3557728: Add GitLab CI pipelines and get all jobs passing I added this to the phpstan-baseline.neon for the project. However, it could be nice to fix this up to properly inject the token service, instead.

Steps to reproduce

Proposed resolution

Use DI to inject the token service for NotifyGroupNode
Remove this case from phpstan-baseline.neon

Remaining tasks

User interface changes

API changes

Data model changes