Better align terminology and labels in prioritization table with handbook guidance

Why is this change being made?

Related to: https://gitlab.com/gitlab-com/Product/-/issues/3316 and !92259 (merged)

This MR further aligns terminology in the master prioritization table to guidance in the product handbook on using the planning priority label for issues important to customers.

This MR also removes the labels customer and customer+ from the prioritization table as they're not necessary or consistently being used to indicate issues important to customers. The labels may still be used and are not being removed from the system, they're just not a signal or required as part of the prioritization table. Usage and definition of these labels is under review and will be clarified elsewhere in the handbook in another iteration.

Author Checklist

• Provided a concise title for the 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.
• Assign reviewers for this change to the correct DRI(s)
  • 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 in 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.
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR.
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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