[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/petstore/java/feign/pom.xml

@@ -319,8 +319,8 @@
         <feign-version>13.2.1</feign-version>
         <feign-form-version>3.8.0</feign-form-version>
         <jackson-version>2.17.1</jackson-version>
-        <jackson-databind-nullable-version>0.2.6</jackson-databind-nullable-version>
         <jackson-databind-version>2.17.1</jackson-databind-version>
+        <jackson-databind-nullable-version>0.2.6</jackson-databind-nullable-version>
         <jakarta-annotation-version>1.3.5</jakarta-annotation-version>
         <junit-version>5.10.0</junit-version>
         <maven-plugin-version>1.0.0</maven-plugin-version>