In 5.0.1 the generated line in `ApiClient` was:
```
return RequestBody.create(MediaType.parse(contentType), (byte[]) obj);
```
in 5.1.0 it is:
```
return RequestBody.create((byte[]) obj, MediaType.parse(contentType));
```
Looks like this change was introduced in #8969, and requires a more recent version of OkHttp to compile in some (all?) circumstances.
My understanding of the version 5 release of OpenAPI generator was that Java 7 support was dropped. Currently the Java client defaults to Java 7, Java 8 can be opted in with the `java8` config option. This removes the conditional logic in `build.gradle` around that config option and defaults to Java 8. This is to support #9151, up-to-date dependency versions rely on Java 8 and it's easier if that is the default.
To be clear, this changes behavior - previous default version was Java 7, this changes it to Java 8.
