Update docs for gitlab-advanced-sast-cpp: List c/c++ as fully supported

What does this MR do and why?

This MR updates docs only, to list c/c++ as fully supported in sast, now that gitlab-advanced-sast-cpp has already been beta released, and will be GA soon.

References

gitlab-advanced-sast-cpp: update SAST docs to i... (#578028 - closed) • Hua Yan • 18.6

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