Drop deprecated SAST-analyzer phpcs-security-audit

phpcs-security-audit is offered as a SAST-analyzer for static security testing of PHP-files: https://docs.gitlab.com/ee/user/application_security/sast/analyzers.html

However, the last update of phpcs-security-audit was on March 2020 (https://github.com/FloeDesignTechnologies/phpcs-security-audit/commits/master) months before PHP 8 was released, no issues have been handled since then. Also the project is looking for a new ownership unsuccessfully for 2 years now (https://github.com/FloeDesignTechnologies/phpcs-security-audit/issues/54).

Therefore the deprecated phpcs-security-audit should be dropped from https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/ci/templates/Jobs/SAST.gitlab-ci.yml

Workaround in own .gitlab-ci.yml

sast:
  variables:
    SAST_EXCLUDED_ANALYZERS: "phpcs-security-audit"