Update phpstan/phpstan-phpunit and make the updated-deps CI job pass again

Problem/Motivation

The (recently revived) updated-deps daily GitLab CI job on 11.x fails PHPStan with a cheery:

 ------ --------------------------------------------------------------------------------- 
  Line   core/modules/system/tests/src/Functional/Render/HtmlResponseAttachmentsTest.php  
 ------ --------------------------------------------------------------------------------- 
  132    Variable $test_meta in empty() always exists and is not falsy.                   
  153    Variable $test_meta in empty() always exists and is not falsy.                   
 ------ --------------------------------------------------------------------------------- 
 ------ ---------------------------------------------------------------------------------- 
  Line   core/modules/taxonomy/tests/src/Kernel/TermKernelTest.php                         
 ------ ---------------------------------------------------------------------------------- 
         Ignored error pattern #^Variable \$depth_count might not be                       
         defined\.$# in path                                                               
         /builds/project/drupal/core/modules/taxonomy/tests/src/Kernel/TermKernelTest.php  
         was not matched in reported errors.                                               
 ------ ---------------------------------------------------------------------------------- 
 ------ ---------------------------------------------------------------------------------------------------------- 
  Line   core/tests/Drupal/KernelTests/Core/Entity/FieldableEntityDefinitionUpdateTest.php                         
 ------ ---------------------------------------------------------------------------------------------------------- 
         Ignored error pattern #^Variable \$entity might not be defined\.$# in                                     
         path                                                                                                      
         /builds/project/drupal/core/tests/Drupal/KernelTests/Core/Entity/FieldableEntityDefinitionUpdateTest.php  
         was not matched in reported errors.                                                                       
 ------ ---------------------------------------------------------------------------------------------------------- 
 [ERROR] Found 4 errors   

Steps to reproduce

Look at the output of the scheduled runs (https://git.drupalcode.org/project/drupal/-/pipeline_schedules) and look at the update-deps jobs

The above failure can be seen at https://git.drupalcode.org/project/drupal/-/jobs/1090935

Proposed resolution

- Bump phpstan/phpstan-phpunit to its latest version (currently 1.3.16)
- Create new PHPStan baseline ($ vendor/bin/phpstan analyze --configuration=core/phpstan.neon.dist --generate-baseline core/.phpstan-baseline.php)
- Set version constraint of phpstan/phpstan-phpunit to its latest version in composer.*, since the new PHPStan baseline wont pass in any lower version.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet