Separate trainee maintainer and reviewer in reviewer roulette

What does this MR do?

Improve code clarity by separating how we retrieve traintainer and reviewers.

From

weighted_reviewers = reviewers[reviewers + traintainers] + hungry_reviewers[reviewers + traintainers] + traintainers + traintainers

to a more explicit

weighted_reviewers = reviewers + hungry_reviewers + traintainers + traintainers + traintainers + hungry_traintainers

This maps more closely to team.yml, where a person is listed as either a reviewer or a trainee maintainer, not both.

Closes #258545 (closed)