Add phpstan (max PHP version) job

Issue information

From user: mscieszka

Related to !169 (merged)

Contribution record

Fork management

Problem/Motivation

Currently, there are composer (max PHP version), nightwatch (max PHP version), phpunit (max PHP version) jobs. PHPStan job can also benefit from using the newer PHP version as the output of the analysis can differ between major PHP versions (for example PHP 8.1 vs PHP 8.2).

Proposed resolution

Create a new phpstan (max PHP version) job.
Update documentation in https://project.pages.drupalcode.org/gitlab_templates/jobs/#when-do-the-jobs-run.
Expand "You can also run the PHPStan job and the tests jobs against previous and/or future versions of Drupal." part of the https://www.drupal.org/project/gitlab_templates page.

Remaining tasks

None.

User interface changes

When OPT_IN_TEST_MAX_PHP = 1 is set in the .gitlab-ci.yml, a new phpstan job appears that uses the $CORE_PHP_MAX version of PHP.