[BLOCKED] Retire the API Review Scoping board anchor (defer to Project 196 sunset)

Part of #45437. BLOCKED: do not start until the Project 196 board retirement is scheduled (https://github.com/orgs/Azure/projects/196/views/19).

## Scope note
This issue originally bundled two things: (1) the atomic rename/flip of the load-bearing stewardship labels, and (2) retiring the `API Review Scoping` board anchor. Part 1 is no longer 196-blocked (the applier is confirmed manual and the Project 1018 view is now live), so it has moved into #45521, which does the auto-apply on the new label and flips all readers in one atomic PR. This issue now covers ONLY part 2.

## Retire the `API Review Scoping` board anchor
`API Review Scoping` is the manual anchor label on the intake/tracking issues (562 of ~619 items on the 196 board; 431 in azure-rest-api-specs, 130 in azure-rest-api-specs-pr). No automation reads it - it exists only to organize the 196 board.

Intake now surfaces on the shared Project 1018 view (#45451, live), which filters on the `data-plane-review-*` labels, so `API Review Scoping` is no longer needed to populate a board. At the 196 retirement: stop applying it and retire it (or leave it only on already-closed historical issues).

Do NOT touch it before the 196 board is actually being retired - the 1018 view covers new intake, but the 196 board still shows historical items via this anchor until it is sunset.