Update Secret Detection direction, including pre-receive

Why is this change being made?

Update Secret Detection direction to:

• Reflect that we are indeed pursuing pre-receive/server-side blocking now, and have made significant progress toward this goal. (Previously it was a "not doing right now" item.)
• Clarify near-term goals:
  • Remove a few, since we can always add them back later if we run out 🙂.
  • Clarify that next-gen is generally prioritized over improvements to current system, but also specifically highlight things we want to do in the current system.
• Tighten up some of the intro text. I deliberately stopped editing this, though, because I can do that separately.

Author and Reviewer Checklist

Please verify the check list and ensure to tick them off before the MR is merged.

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added, and the content is SAFE
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies
  • The when to get approval handbook section explains the workflow in more detail
• For transparency, share this MR with the audience that will be impacted.
  • Team: For changes that affect your direct team, share in your group Slack channel
  • Department: If the update affects your department, share the MR in your department Slack channel
  • Company: If the update affects all (or the majority of) GitLab team members, post an update in #whats-happening-at-gitlab linking to this MR
    • For high-priority company-wide announcements work with the internal communications team to post the update in #company-fyi and align on a plan to circulate in additional channels like the "While You Were Iterating" Newsletter

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