chore: ensure roulette picks maintainers properly

What does this MR do?

According to the handbook, every frontend engineer is a reviewer of GitLab UI, but not all of then are marked as such in the team.yml. Similarly, not all GitLab maintainers are marked as GitLab UI maintainers. This results in some discrepancies in the reviewers & maintainers lists between GitLab and GitLab UI, as can be seen in https://about.gitlab.com/handbook/engineering/projects/.

For this reason, the reviewers roulette currently pulls data from GitLab's team, which better represents the workforce, but it's not perfect either. For example, I'm myself a maintainer of GitLab UI, but not of GitLab, which means that the roulette still considers me as a reviewer.

To address this, this MR changes the roulette's logic to pull data from both GitLab and GitLab UI. The reviewers are then selected as follows:

• A reviewer is anyone that's either a GitLab reviewer, a GitLab UI reviewer, or a trainee maintainer but isn't a GitLab UI maintainer.
• A traintainer is anyone that's a trainee maintainer of either GitLab or GitLab UI.
• A maintainer is anyone that's a maintainer of either GitLab or GitLab UI.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • CustomersDot: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• Security reports checked/validated by a reviewer from the AppSec team

Closes #1110 (closed)