unparser: use to_rfc3339 for TIMESTAMPTZ formatting #21294

/// Returns an RFC 3339 and ISO 8601 date and time string such as `1996-12-19T16:39:57-08:00`.
    #[cfg(feature = "alloc")]
    #[must_use]
    pub fn to_rfc3339(&self) -> String {
        // For some reason a string with a capacity less than 32 is ca 20% slower when benchmarking.
        let mut result = String::with_capacity(32);
        let naive = self.overflowing_naive_local();
        let offset = self.offset.fix();
        write_rfc3339(&mut result, naive, offset, SecondsFormat::AutoSi, false)
            .expect("writing rfc3339 datetime to string should never fail");
        result
    }