Refactor semgrep-appsec-custom-rules to not fetch

What does this MR do and why?

• Refactor semgrep-appsec-custom-rules to not git fetch origin master. This fetch appears to be unnecessary.
• Update semgrep version from 1.99.0 to 1.123.0

This fixes issues that we see where jobs have an inflated runtime (1m vs ~11m) This fixes issues that we see where jobs fail for unrelated reasons. (Example failed job)

Actions post-merge

• Backport to 18.0
• Backport to 17.11
• Backport to 17.10
• Backport to 17.9

References

gitlab-com/gl-infra/delivery#21126 (closed)

Screenshots or screen recordings

✅ was able to verify this works in https://gitlab.com/gitlab-org/gitlab/-/jobs/10193877147#L107

That job scanned 2 ruby files that were changed in the MR.

How to set up and validate locally

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