fix(postgres): regression of #1449

```
error: error occurred while decoding column 0: data did not match any variant of untagged enum Explain at line 3 column 1
Error:    --> tests/postgres/macros.rs:103:15
    |
103 |     let row = sqlx::query!(r#"CALL forty_two(null)"#)
    |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: this error originates in the macro `$crate::sqlx_macros::expand_query` which comes from the expansion of the macro `sqlx::query` (in Nightly builds, run with -Z macro-backtrace for more info)

error: could not compile `sqlx` (test "postgres-macros") due to previous error
```

sqlx-postgres/src/connection/describe.rs

@@ -494,8 +494,7 @@ enum Explain {
     QueryPlan(QueryPlan),
     /// The string "Utility Statement" -- returned for
     /// a CALL statement
-    #[serde(rename = "Utility Statement")]
-    UtilityStatement,
+    UtilityStatement(String),
 }
 
 #[derive(serde::Deserialize)]