Docs: Add footnote to .NET Framework support in SAST

What does this MR do?

Add a footnote to the SAST Supported Languages and Frameworks table. It is currently tribal knowledge that .NET 4 may or may not work, but we should make this clearer.

I didn't remove the table row outright because I have seen some reports of .NET 4 projects working in limited circumstances. I don't have references handy.

Related issues

This came up in #349615 (comment 989837650) but has surfaced in many other places in Slack, email, and issues before.

Also relates to #351198 (closed).

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