Change admin repo to allow maintainers (who are the approvers in that repo) to merge PRs

[trask]

This would enable maintainers to self-service their own repo configuration without needing to wait for someone in @open-telemetry/admins to merge it.

Changes that apply to multiple files will require approval from all of those applicable maintainers (listed in CODEOWNERS), or @open-telemetry/admins (which is CODEOWNER for everything), so I don't think that giving this additional permission is overly broad.

And @open-telemetry/admins can always follow-up afterwords if there is any concern about a specific change.