Docs: Remove obsolete Semgrep transition steps, simplify translation

What does this MR do?

• Remove some very-old content about activating Semgrep-based scanning early
  • Now it's on by default so this is not important.
• Simplify the description of which vulns are translated, since we have a lot more analyzers to discuss now, but the details are actually hard to understand anyway.
  • Seems better to just say "we try, and here are reasons it may not work".

Related issues

• Update "Transition to Semgrep-based scanning" s... (#382156 - closed) • Connor Gilbert • 17.2
• Explain how findings are handled when Semgrep-b... (#412795 - closed) • Connor Gilbert • 17.2

Also relevant to https://gitlab.com/gitlab-com/gitlab-OKRs/-/issues/8023+.

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