[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/feign-gson/build.sbt

@@ -12,15 +12,9 @@ lazy val root = (project in file(".")).
       "io.swagger" % "swagger-annotations" % "1.6.11" % "compile",
       "com.google.code.findbugs" % "jsr305" % "3.0.2" % "compile",
       "io.github.openfeign" % "feign-core" % "10.12" % "compile",
-      "io.github.openfeign" % "feign-jackson" % "10.12" % "compile",
       "io.github.openfeign" % "feign-slf4j" % "10.12" % "compile",
       "io.github.openfeign.form" % "feign-form" % "3.8.0" % "compile",
       "io.github.openfeign" % "feign-okhttp" % "10.12" % "compile",
-      "com.fasterxml.jackson.core" % "jackson-core" % "2.17.1" % "compile",
-      "com.fasterxml.jackson.core" % "jackson-annotations" % "2.17.1" % "compile",
-      "com.fasterxml.jackson.core" % "jackson-databind" % "2.17.1" % "compile",
-      "com.fasterxml.jackson.datatype" % "jackson-datatype-jsr310" % "2.17.1" % "compile",
-      "com.github.joschi.jackson" % "jackson-datatype-threetenbp" % "2.15.2" % "compile",
       "com.github.scribejava" % "scribejava-core" % "8.0.0" % "compile",
       "com.brsanthu" % "migbase64" % "2.2" % "compile",
       "jakarta.annotation" % "jakarta.annotation-api" % "1.3.5" % "compile",