github/CODEOWNERS: rename code owners file to soft deactivate

In this commit, rather than attempt to reduce the scope of the current
`CODEOWNERS` file, we instead rename it as a way to stop Github from
automatically assigning reviewers based on it. Instead, we put things in
a "hint" mode as a way for PR OPs to figure out who they should manually
assign PRs today.

With the CODEOWNERS file reduced to a code review assignment hint, we
aim to instead promote usage of the `@code-review` team which will use a
load balancing algorithm for automatic code review assignment.

Author: Roasbeef