Add @keithmattix as a codeowner maintainer (#4861)

@keithmattix has been contributing to the project at the level that is
expected of a maintainer to include: Designs and implementations of
feature work, complex bug triaging and fixing, and indepth and sustained
code reviews raising the quality of the product.

**Maintainers**, if you agree with this nomination, leave a "+1" comment
and approve the PR (do not merge it).

Signed-off-by: Thomas Stringer <[email protected]>

Author: trstringer

CODEOWNERS

@@ -1,6 +1,6 @@
 # ref: https://docs.github.com/en/github/creating-cloning-and-archiving-repositories/about-code-owners
 
-* @shashankram @snehachhabria @nojnhuh @draychev @jaellio @trstringer
+* @shashankram @snehachhabria @nojnhuh @draychev @jaellio @trstringer @keithmattix
 
 # Emeritus maintainers
 # @michelleN @eduser25

OWNERS

@@ -6,6 +6,7 @@
 - @draychev
 - @jaellio
 - @trstringer
+- @keithmattix
 
 # Non-code maintainers
 - @phillipgibson