Update release/6.0 CODEOWNERS to branch managers only (#7625)

Author: bnbarham

CODEOWNERS

@@ -1,27 +1,2 @@
-# This file is a list of the people responsible for ensuring that patches for a
-# particular part of Swift are reviewed, either by themselves or by someone else.
-# They are also the gatekeepers for their part of Swift, with the final word on
-# what goes in or not.
-#
-# The list is sorted by surname and formatted to allow easy grepping and
-# beautification by scripts.  The fields are: name (N), email (E), web-address
-# (W), PGP key ID and fingerprint (P), description (D), and snail-mail address
-# (S).
-
-# N: Tomer Doron
-# E: [email protected]
-# D: Package Manager
-
-# N: Max Desiatov
-# E: [email protected]
-# D: Package Manager
-
-# N: Ben Barham
-# E: [email protected]
-# D: Package Manager
-
-###
-
-# The following lines are used by GitHub to automatically recommend reviewers.
-
-* @bnbarham @MaxDesiatov @tomerd
+# For the release branch @apple/swift-branch-managers needs to approve the changes
+* @apple/swift-branch-managers