Docs: Remove SAST CI/CD configs for end-of-support analyzers

What does this MR do?

Remove CI/CD variable settings that only apply to analyzers that have reached End of Support. For context, see deprecation announcements:

• 17.0
• 16.0
• 15.4

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

Author's checklist

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Reviewer's checklist

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