CODEOWNERS: Rework code owner entries
CODEOWNERS: Rework code owner entries

This commit reworks the code owner entries, mostly to avoid matching the
catch-all janitors team unless it makes sense. In particular, it
- fixes a few entries in api/, pkg/option/, pkg/service/, and
  install/kubernetes/.
- introduces a new team cilium/build;
- moves development tools under cilium/contributing;
- removes specific code owners to prefer teams;
- adds go.{sum,mod} to cilium/vendor.

Signed-off-by: Paul Chaignon <a027184a55211cd23e3f3094f1fdc728df5e0500@cilium.io>

michi-covalent/cilium,tgraf/cilium,cilium/cilium,cilium/cilium,cilium/cilium,tklauser/cilium,tklauser/cilium,michi-covalent/cilium,michi-covalent/cilium,cilium-team/cilium,tklauser/cilium,michi-covalent/cilium,tgraf/cilium,tgraf/cilium,tklauser/cilium,cilium/cilium,tgraf/cilium,cilium-team/cilium,tgraf/cilium,tklauser/cilium,michi-covalent/cilium,cilium/cilium,tgraf/cilium