Soft-deprecate the fast quarantine process

Context

We don't need the fast-quarantine process as much as we used to when we rolled it out, because we now have an efficient workflow with the pipelineexpedited label.

Closes gitlab-org/quality/engineering-productivity/team#518 (closed)

What does this MR do?

• Soft-deprecate the fast quarantine process by mentioning pipelineexpedited as a preferred alternative
• Add some explanation about the need to always long-quarantine after fast-quarantining.
  • Also done in gitlab-org/quality/engineering-productivity/fast-quarantine!196 (merged)

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