Clippy fix for Rust 1.63 (#6299)

Take more sensitive lints into account

Somewhat ugly in some cases is the replacement of `.get(0)` with
`.first()`

crates/nu-command/src/conversions/into/datetime.rs

@@ -311,7 +311,7 @@ fn action(
                 Some(dt) => match DateTime::parse_from_str(val, &dt.0) {
                     Ok(d) => Value::Date { val: d, span: head },
                     Err(reason) => {
-                        return Value::Error {
+                        Value::Error {
                             error: ShellError::CantConvert(
                                 format!("could not parse as datetime using format '{}'", dt.0),
                                 reason.to_string(),