Add iwiedler as trainee_maintainer for gitlab-runner project

Summary

As part of the runners platform team we are co-maintaining this code base with runners core.

Contributions thus far:

• https://gitlab.com/gitlab-org/gitlab-runner/-/merge_requests/?sort=created_date&state=all&author_username=igorwwwwwwwwwwwwwwwwwwww&first_page_size=20

This is the first step towards full maintainership.