Update CODEOWNERS

@brandjon remains an admin in the repository but @tetromino is the active maintainer.

CODEOWNERS

diff --git a/CODEOWNERS b/CODEOWNERS
index 9858bec..6be0415 100644
--- a/CODEOWNERS
+++ b/CODEOWNERS
@@ -1 +1 @@
-* @brandjon @tetromino
+* @tetromino