Docs: Remove beta wording ahead of the 18.8 GA release of gitlab-advanced-sast-cpp

What does this MR do and why?

Since gitlab-advanced-sast-cpp is about to reach GA in 18.8, this MR removes the “beta” wording.

References

gitlab-advanced-sast-cpp: Remove mention of Bet... (#578435) • Hua Yan • 18.8

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