rename vault namespace env variable to be more idiomatic (#6905)

This commit renames the env variable for vault namespaces
such that it begins with `MINIO_SSE_`. This is the prefix
for all Minio SSE related env. variables (like KMS).

cmd/crypto/vault.go

@@ -193,7 +193,7 @@ func NewVault(kmsConf KMSConfig) (KMS, error) {
 		return nil, err
 	}
 
-	if ns, ok := os.LookupEnv("VAULT_NAMESPACE"); ok {
+	if ns, ok := os.LookupEnv("MINIO_SSE_VAULT_NAMESPACE"); ok {
 		c.SetNamespace(ns)
 	}