Assign trainee maintainer role to Pedro Pombeiro

Why is this change being made?

I'm creating this issue as part of applying to become a trainee maintainer (#6840 (closed)). I'm not sure if I should add myself as trainee_maintainer for gitlab-runner only or for autoscaler as well.

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what