Reduce oncall top-level reviewers.
Reduce oncall top-level reviewers.

Oncall has people with different amounts of involvement in test-infra.
It is useful to have oncall be able to approve these PRs when necessary,
but we should try to direct reviews to the people actively involved in
this repo. So reduce to that subset.

michelle192837/test-infra,michelle192837/test-infra,michelle192837/test-infra,dims/test-infra,fejta/test-infra,cjwagner/test-infra,dims/test-infra,monopole/test-infra,cblecker/test-infra,cblecker/test-infra,BenTheElder/test-infra,monopole/test-infra,jessfraz/test-infra,monopole/test-infra,monopole/test-infra,kubernetes/test-infra,dims/test-infra,jessfraz/test-infra,pwittrock/test-infra,kubernetes/test-infra,jessfraz/test-infra,fejta/test-infra,kubernetes/test-infra,michelle192837/test-infra,pwittrock/test-infra,cjwagner/test-infra,pwittrock/test-infra,cblecker/test-infra,jessfraz/test-infra,BenTheElder/test-infra,dims/test-infra,BenTheElder/test-infra,cblecker/test-infra,BenTheElder/test-infra,monopole/test-infra,kubernetes/test-infra,cblecker/test-infra,fejta/test-infra,cblecker/test-infra,jessfraz/test-infra,BenTheElder/test-infra,cjwagner/test-infra,BenTheElder/test-infra,cjwagner/test-infra,dims/test-infra,michelle192837/test-infra,dims/test-infra,michelle192837/test-infra,fejta/test-infra,cjwagner/test-infra,kubernetes/test-infra,pwittrock/test-infra,fejta/test-infra,fejta/test-infra,kubernetes/test-infra,jessfraz/test-infra,monopole/test-infra,cjwagner/test-infra,pwittrock/test-infra