[JAVA: okhttp-gson, rest-assured, retrofit2] Don't generate Jackson import when serialization library is GSON (#18811)

* Partially revert "replace deprecated ISO8601Utils with StdDateFormat (#17052)"

This partially reverts commit 76560e34c9, namely anything
related to generators and samples using GSON instead of Jackson.

Changes to Jackson-only generation and generator-online regarding RFC3339DateFormat
are not being reverted.

* Test for default serialization library fallback

* Convert repetitive tests to parameterized test

* Add regression test for #18515

* [FEIGN] Only include <jackson-databind-version> property in pom.xml when required

* [RETROFIT2] Only include jackson-databind in gradle file when actually required

* [FEIGN] Don't include jackson dep's in sbt file when GSON is selected

* [FEIGN] Don't include jackson dep's in gradle file when GSON is selected

* DRY refactor JavaClientCodegen test code, increase readability

- use fluent assertions
- use helper method newTempFolder()
- use Java 9 static factory methods for maps
- don't declare variables that are only used once
- group declarations and usages
- use non-blocking java.nio.file API wherever possible

* Regenerate samples

samples/client/echo_api/java/okhttp-gson/src/main/java/org/openapitools/client/JSON.java

@@ -13,11 +13,11 @@
 
 package org.openapitools.client;
 
-import com.fasterxml.jackson.databind.util.StdDateFormat;
 import com.google.gson.Gson;
 import com.google.gson.GsonBuilder;
 import com.google.gson.JsonParseException;
 import com.google.gson.TypeAdapter;
+import com.google.gson.internal.bind.util.ISO8601Utils;
 import com.google.gson.stream.JsonReader;
 import com.google.gson.stream.JsonWriter;
 import com.google.gson.JsonElement;
@@ -31,16 +31,14 @@ import java.io.StringReader;
 import java.lang.reflect.Type;
 import java.text.DateFormat;
 import java.text.ParseException;
+import java.text.ParsePosition;
 import java.time.LocalDate;
 import java.time.OffsetDateTime;
-import java.time.ZoneId;
-import java.time.ZoneOffset;
 import java.time.format.DateTimeFormatter;
 import java.util.Date;
 import java.util.Locale;
 import java.util.Map;
 import java.util.HashMap;
-import java.util.TimeZone;
 
 /*
  * A JSON utility class
@@ -57,11 +55,6 @@ public class JSON {
     private static LocalDateTypeAdapter localDateTypeAdapter = new LocalDateTypeAdapter();
     private static ByteArrayAdapter byteArrayAdapter = new ByteArrayAdapter();
 
-    private static final StdDateFormat sdf = new StdDateFormat()
-        .withTimeZone(TimeZone.getTimeZone(ZoneId.systemDefault()))
-        .withColonInTimeZone(true);
-    private static final DateTimeFormatter dtf = DateTimeFormatter.ISO_OFFSET_DATE_TIME;
-
     @SuppressWarnings("unchecked")
     public static GsonBuilder createGson() {
         GsonFireBuilder fireBuilder = new GsonFireBuilder()
@@ -342,7 +335,7 @@ public class JSON {
                         if (dateFormat != null) {
                             return new java.sql.Date(dateFormat.parse(date).getTime());
                         }
-                        return new java.sql.Date(sdf.parse(date).getTime());
+                        return new java.sql.Date(ISO8601Utils.parse(date, new ParsePosition(0)).getTime());
                     } catch (ParseException e) {
                         throw new JsonParseException(e);
                     }
@@ -352,7 +345,7 @@ public class JSON {
 
     /**
      * Gson TypeAdapter for java.util.Date type
-     * If the dateFormat is null, DateTimeFormatter will be used.
+     * If the dateFormat is null, ISO8601Utils will be used.
      */
     public static class DateTypeAdapter extends TypeAdapter<Date> {
 
@@ -377,7 +370,7 @@ public class JSON {
                 if (dateFormat != null) {
                     value = dateFormat.format(date);
                 } else {
-                    value = date.toInstant().atOffset(ZoneOffset.UTC).format(dtf);
+                    value = ISO8601Utils.format(date, true);
                 }
                 out.value(value);
             }
@@ -396,7 +389,7 @@ public class JSON {
                             if (dateFormat != null) {
                                 return dateFormat.parse(date);
                             }
-                            return sdf.parse(date);
+                            return ISO8601Utils.parse(date, new ParsePosition(0));
                         } catch (ParseException e) {
                             throw new JsonParseException(e);
                         }