doc: mark Node.js v15.x as EOL
Today is 2021-06-01, which is the date v15.x release line is marked as End-of-Life.
Refs: https://github.com/nodejs/Release/blob/main/schedule.json
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Today is 2021-06-01, which is the date v15.x release line is marked as End-of-Life.
Refs: https://github.com/nodejs/Release/blob/main/schedule.json