chore: update CODEOWNERS config

[JustinBeckwith]

No description provided.

[codecov]

Codecov Report

Merging #727 (87e59a2) into master (8dab7bf) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #727   +/-   ##
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  Coverage   87.13%   87.13%           
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  Files          55       55           
  Lines        6683     6683           
  Branches      616      616           
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  Hits         5823     5823           
  Misses        859      859           
  Partials        1        1           

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[bcoe]

@JustinBeckwith should this include a CODEOWNERS file?

[JustinBeckwith]

So ... we have a CODEOWNERS here!
https://github.com/googleapis/release-please/blob/master/.github/CODEOWNERS

I am trying to move us to a model similar to what happens in Java, where it's generated from the .repo-metadata.json:
googleapis/synthtool#936

I wanted to land the .repo-metadata.json changes first, and then let synthtool handle any cleanup along the way.