fix nats TLS unit tests (#10476)

This commit fixes the nats TLS tests by generating new certificates
(root CA, server and client) - each valid for 10y. The new certificates
don't have a common name (deprecated by X.509) but SANs instead.

Since Go 1.15 the Go `crypto/x509` package rejects certificates that
only have a common name and no SAN. See: https://golang.org/doc/go1.15#commonname

pkg/event/target/testdata/nats_tls_client_cert.conf

@@ -13,6 +13,6 @@ authorization {
         subscribe = ">"
     }
     users = [
-        {user: "CN=localhost,OU=Client,O=Minio,C=CA", permissions: $ADMIN}
+        {user: "CN=localhost,OU=Client,O=MinIO,C=CA", permissions: $ADMIN}
     ]
 }