ci: add renovate reviewers for files not matched on CODEOWNERs

Follow-on from !4711 (merged)

After reviewing the first round of version updates proposed by Renovate its become clear that using CODEOWNERS isn't going to be sufficient.

Many of the upgrades are on files that won't make sense to have the Renovate reviewers as CODEOWNERS on (for example, .gitlab-ci.yml). Additionally there is a long-tail of files that could receive upgrades, and adding them all to CODEOWNERS is tedious.

This manually configures renovate to select from the list of engineers who have volunteered for this task.

Side note: I've also gone ahead and done some tweaks to our common Renovate configuration (see gitlab-com/gl-infra/common-ci-tasks!133 (merged) and gitlab-com/gl-infra/common-ci-tasks!132 (merged)) to improve on the Renovate proposed upgrades. More to follow.