chore: add deepgram-kiley and dg-coreylweathers to CODEOWNERS (#732)

Adds `@deepgram-kiley` and `@dg-coreylweathers` as global code owners
alongside `@GregHolmes`.

```diff
- * @GregHolmes
+ * @GregHolmes @deepgram-kiley @dg-coreylweathers
```

Note: the new owners must have repo/org access for GitHub to honor them
on review requests.