Reduce the frequency of trainee maintainers in Reviewer Roulette

From the [2022-04-25 Maintainership working group](https://docs.google.com/document/d/1RuWhO2q7rsgSKrnSCf2xsOrp56SXBHe_z5XKDls5px8/edit#) we discussed that maintainers and trainees sometimes take their busy indicator off, and immediately get several reviews. Because trainee maintainers get 3 times the amount of a regular reviewer (so that they get more experience), this can be even more overwhelming. We should consider reducing this.


### Context

Initially, [trainees were weighted higher](https://gitlab.com/gitlab-org/gitlab-foss/-/issues/60035) at 3x as a trial but it does not look like we ever followed up. At the time, this would have meant a trainee maintainer gets picked 3/39 of the time.

Today, [Roulette works by](https://gitlab.com/gitlab-org/ruby/gems/gitlab-dangerfiles/blob/master/lib/danger/plugins/roulette.rb#L147) creating a list of reviewers, traintainers, and maintainers. Since traintainers _are_ reviewers, this means they are added to the list 4x [(3x for being a traintainer and 1 more time for being a reviewer)](https://gitlab.com/gitlab-com/www-gitlab-com/-/issues/13298#note_924545182). 

Does this still make sense?

### Data

Here is a brief sample of preliminary data:

For frontend:
- 25 maintainers
- 11 trainee maintainers
- 21 reviewers (10 non-trainees)
- Chances of a regular reviewer getting picked: 1/21 or 4% of the time
- Chances of a trainee maintainer getting picked: 4/21 or 19% of the time
- Chances of a maintainer getting picked: 1/25 or 4% of the time

For backend:
- 44 maintainers
- 19 trainee maintainers
- 55 reviewers (36 non-trainees)
- Chances of a regular reviewer getting picked: 1/55 or 2% of the time
- Chances of a trainee maintainer getting picked: 4/55 or 7% of the time
- Chances of a maintainer getting picked: 1/44 or 2% of the time

For database:
- 9 maintainers
- 5 trainee maintainers
- 19 reviewers (14 non-trainees)
- Chances of a regular reviewer getting picked: 1/19 or 5% of the time
- Chances of a trainee maintainer getting picked: 4/19 or 21% of the time
- Chances of a maintainer getting picked: 1/9 or 11% of the time

### Resources
- [Roulette logic](https://gitlab.com/gitlab-org/ruby/gems/gitlab-dangerfiles/-/blob/master/lib/danger/plugins/roulette.rb)
- [Roulette Dangerfile](https://gitlab.com/gitlab-org/gitlab/-/blob/master/danger/roulette/Dangerfile)

### Plan

- [x] Reduce trainee maintainers weight in Roulette - see https://gitlab.com/gitlab-com/www-gitlab-com/-/issues/13298#note_972867144
   - https://gitlab.com/gitlab-org/ruby/gems/gitlab-dangerfiles/-/merge_requests/132, https://gitlab.com/gitlab-org/ruby/gems/gitlab-dangerfiles/-/merge_requests/134 and https://gitlab.com/gitlab-org/gitlab/-/merge_requests/90389
   - new Roulette calculation
   ```
      # +------------------------------+--------------------------------+
      # |        reviewer type         | weight(times in reviewer pool) |
      # +------------------------------+--------------------------------+
      # | reduced capacity reviewer    |                              1 |
      # | reviewer                     |                              2 |
      # | hungry reviewer              |                              4 |
      # | reduced capacity traintainer |                              1 |
      # | traintainer                  |                              2 |
      # | hungry traintainer           |                              6 |
      # +------------------------------+--------------------------------+
   ```
- [x] Communicate this change in EWIR(engineering week in review) and development slack channel

Story points: 3.0