Remove dashboard code_ref compatibility after promotion

[trask]

After the first pull request dashboard release containing #285 is promoted to the stable channel, remove the temporary code_ref compatibility wiring.

• Remove the ignored workflow_call.inputs.code_ref input from pull-request-dashboard-repo.yml.
• Remove code_ref from the three stable jobs in pull-request-dashboard.yml.
• Simplify promote_rollout.py and its tests so promotion updates only the reusable-workflow pin.
• Remove the transition notes from the rollout documentation.

This must wait until that promotion because the currently stable v0.10.0 workflow still uses code_ref to check out its matching dashboard scripts.