Update location of custom SAST rules

What does this MR do and why?

We (AppSec) are doing some reorganization of our custom rules project. The old rules have been copied in gitlab-com/gl-security/appsec/sast-custom-rules@34273247 to avoid breaking the semgrep-sast job and the master build. When this is merged the previous location of the rules will be deleted and the "appsec-pings" directory will be used for something else (things AppSec wants to be pinged on but shouldn't show up in the Vulnerability Report).

Screenshots or screen recordings

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How to set up and validate locally

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