Docs: Simplify SAST requirements, remove Docker 19.03.00 warning

What does this MR do?

• Remove warning about Docker 19.03.0 (#13830 (comment 211354992)) from the main SAST and SD docs pages.
  • This version is from 2019 (19.x) and seems to have been resolved nearly immediately afterward (19.03.01).
  • The relevant troubleshooting content also remains in SAST and DS documentation.
  • So, essentially the change is to stop including this "by default" when people look at the requirements.
• Change the structure of the SAST requirements section to match SD's section, which covers the "you can only use amd64" requirement more elegantly.

Related issues

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

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