SAST Docs: Remove Go proxy instructions, update precompilation

What does this MR do?

1. Update the pre-compilation section and the table entry for the COMPILE option to refer only to SpotBugs. (Previously it was applicable to SpotBugs and Gosec, but Gosec was removed in 15.4.)
2. Remove a section that was relevant only to Gosec.

I believe this is OK under the guidelines for documenting product versions, because the docs are meant to reflect current state. I'm hoping to simplify the page as much as possible, in part due to the attached OKR.

Related issues

https://gitlab.com/gitlab-com/gitlab-OKRs/-/issues/8023

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