Introduce a `~"workflow::complete` label to the product development workflow

Problem

We do not have a consistent way to filter by completed issues.

Some teams use workflow::verification, others use workflow::production, and many issues get closed with workflow:: labels that do not reflect the true state of the work (example).

The lack of adherence to workflow:: labels on closed issues results in confusion for users and internal stakeholders. For example, linking to a filtered list of Bugs, Performance Improvements, etc. by milestone isn't possible in our current state.

Enforcing label application on closed issues with triage-ops would be an improvement, but we don't have an agreement on what label should be applied to a completed issue.

Prior suggestion

!117215 (closed) suggested that workflowproduction should represent the end of the workflow. However, there are examples of why this would not be a clean change (example, example).

~workflow::complete will be the unified end of the line

This MR introduces ~workflow::complete as the end of the line for verified work in production. (additional context)

Enables label recommendations and enforcement via triage-ops

Similar to the way the Work Type Classification labels allow for automated label recommendations and more reliable metrics, this change would allow us to address the inconsistency of workflow application labels by establishing a common expectation for all complete and closed issues.

Any closed issue that does not include a ~workflow::complete or ~closed::* label can be pinged via bot for clarification.

Any closed issue with ~workflow::complete can have ~closed::complete added via a bot for reporting.

Changes in this MR

• Updates the product development workflow handbook page to reflect that completed issues should be Closed and have the workflow::complete label applied.
• Updates occurrences in the handbook where workflowverification or workflowproduction indicate completion

Follow-up

• Add ~"workflow::complete" to gitlab and www-gitlab-com
• Consolidate various closed labels into closed:: scope #14410 (closed)

Author Checklist

• 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
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• 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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