Java Convert Time to iso_instant

Your date-time string has timezone Id instead of timezone offset and therefore you should parse it into ZonedDateTime. Also, Never use SimpleDateFormat or DateTimeFormatter without a Locale.

import java.time.Instant;
import java.time.ZonedDateTime;
import java.time.format.DateTimeFormatter;
import java.util.Locale;

public class Main {
    public static void main(String[] args) {
        DateTimeFormatter dtf = DateTimeFormatter.ofPattern("EEE MMM dd HH:mm:ss zzz uuuu", Locale.ENGLISH);

        Instant instant = ZonedDateTime.parse("Thu Feb 04 19:03:27 CET 2021", dtf).toInstant();
        System.out.println(instant);

        instant = ZonedDateTime.parse("Sat Oct 03 11:57:00 CEST 2020", dtf).toInstant();
        System.out.println(instant);
    }
}

Output:

2021-02-04T18:03:27Z
2020-10-03T09:57:00Z

Learn more about java.time, the modern date-time API^* from Trail: Date Time.

^{* For any reason, if you have to stick to Java 6 or Java 7, you can use ThreeTen-Backport which backports most of the java.time functionality to Java 6 & 7. If you are working for an Android project and your Android API level is still not compliant with Java-8, check Java 8+ APIs available through desugaring and How to use ThreeTenABP in Android Project.}