Adjust SAST exists rule for php

What does this MR do and why?

Adjusts an exists rule that was added in Add PHP language support to gitlab-advanced-sast (!185133 - merged) • Julian Thome, Philip Cunningham • 18.1.

References

• https://gitlab.com/gitlab-org/gitlab/-/issues/523657+s
• Refactor php-related SAST rules (#559997 - closed) • Adam Cohen • 18.4

Manual testing

When a single *.php file exists and GITLAB_ADVANCED_SAST_ENABLED: true is set:

• Before this change:

  Both gitlab-advanced-sast and semgrep-sast jobs are present, while only the gitlab-advanced-sast job should be present. pipeline

• After this change:

  Only a single gitlab-advanced-sast job is present, as expected. pipeline