add simplified CODEOWNERS file for older release branches

.gitlab/CODEOWNERS

@@ -0,0 +1,6 @@
+# For the syntax of this file, see:
+#
+#    https://docs.gitlab.com/ee/user/project/code_owners.html#the-syntax-of-code-owners-files
+#
+
+* @esp-idf-codeowners/all-maintainers