Remove outdated caveats about "blocked" pipelines

What does this MR do?

In !133197 (540c1fe8) we clarified that special circumstances apply to a Pipeline's Security tab, but during Allow security reports to be read for pipelines... (#439691 - closed) that bevaiour was aligned and updated (see esp. !146379 (c99b0a49)).

Therefore, should we focus the relevant docs sections to the current/harmonized/standard behavior, removing duplicated mentions of those out-dated caveats?

Since we'll keep the 16.10 & 16.11 versions on that page for a while, it seems better to sometimes refer to that, rather than carrying multiple 16.y mentions forward.

Re the cancelled pipeline: I intend to use this as a test case for local, GDK-based linting, so let's focus the MR review on content for now. I'll push any style updates, as make gitlab-docs-check finds them locally.

Related issues

• Allow security reports to be read for pipelines... (#439691 - closed)
• Post merge doc review for ingest scan reports f... (#443783)

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
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Reviewer's checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
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    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
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