chore: rename OWNERS to OWNERS.json5

> OWNERS files support JSON5 syntax (see json5.org), a more
> human-readable and writable variant of JSON which supports
> comments, unquoted object keys, single quotes, and trailing
> commas. 

All files should have their correct extensions.

Refs: https://json5.org

Signed-off-by: Derek Lewis <dereknongeneric@open.inf.is>

Author: DerekNonGeneric