This removes most `CODEOWNERS` entry other than to a few parts of the
codebase that are security-sensitive.
The rest of the metadata in `areas.yaml` is still useful for the
purpose of PR auto-assignment, but it needs not be as strongly gated
as `CODEOWNERS` implies...
This expands the `maintainers.yaml` file to contain the notion of
"areas", to which maintainers are assigned. That then is used to
generate the `CODEOWNERS` file.
Updates issue #14095.
