Update v26 start date

[haines]

nodejs/node#62526 (comment)
nodejs/node#62526 (comment)

[MikeMcC399]

https://github.com/nodejs/node/releases/tag/v26.0.0 was released on May 5, 2026, so there shouldn't be any doubt now about the correctness of merging this change to the release date.

The https://github.com/nodejs/release#release-schedule table should also be extended to make the dates for Node.js 26.x more easily viewable, instead of relying just on the data in https://github.com/nodejs/Release/blob/main/schedule.json

[richardlau]

The https://github.com/nodejs/release#release-schedule table should also be extended to make the dates for Node.js 26.x more easily viewable, instead of relying just on the data in https://github.com/nodejs/Release/blob/main/schedule.json

Opened #1156