[Chore] Update CODEOWNERS for new CI infrastructure
`.buildkite` is gone, replaced with `.expeditor`
Also went ahead and removed the line for `components/backline`, since
a) there were no names anyway, and b) there's not really any code to
that component.
-.buildkite @christophermaier
-components/backline
+.expeditor @christophermaier @smacfarlane
components/builder-api-client @chefsalim @raskchanky
components/butterfly* @christophermaier @raskchanky
components/core @christophermaier @chefsalim @raskchanky
