Align 6-week cycle check with beta promotion instead of stable release.

Align 6-week cycle check with beta promotion instead of stable release.

The regression check is to make beta promotion easier, so it makes more
sense to use the Tuesday of the release week (T-2) as the end point of the
regression prevention, instead of Thursday (T-0). But since the beta
promotion PR is sent at Tuesday evening at UTC, the protection should
include the whole Tuesday as well, meaning the 6-week cycle will start from
Wednesdays.

This will also move the start of the regression protection week one day
earlier.