Update golang toolchain version to 1.23.9 (#1034)

Author: dhurley

go.mod

@@ -2,7 +2,7 @@ module github.com/nginx/agent/v3
 
 go 1.23
 
-toolchain go1.23.2
+toolchain go1.23.8
 
 require (
 	buf.build/gen/go/bufbuild/protovalidate/protocolbuffers/go v1.33.0-20240401165935-b983156c5e99.1