[kotlin] fix Date types usages

[4brunu]

AbstractKotlinCodegen.java has some issues related to date types.

The first one is that in the typeMapping it defines Date as java.time.LocalDate which is correct.
But then in importMapping it defines Date as java.time.LocalDate which is incorrect and doesn't match with the typeMapping
It should define Date as java.time.LocalDate in the typeMapping.

The second one is that it defines DateTime as java.time.LocalDateTime in the typeMapping and in the importMapping, which is wrong, it should be java.time.OffsetDateTime.
This can cause bugs because the servers and the clients can (and most probably are) in different timezones.
This is already fixed in KotlinSpringServerCodegen and KotlinClientCodegen but not in AbstractKotlinCodegen.

Finally since java.util.Date is never used, it's not necessary in the Kotlin Client Serializer, so it's removed.

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@jimschubert (2017/09) ❤️, @dr4ke616 (2018/08) @karismann (2019/03) @Zomzog (2019/04) @andrewemery (2019/10) @4brunu (2019/11) @yutaka0m (2020/03)

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[4brunu]

This PR has lots of changes, because I generate the samples server and openapi3, which were really outdated, because they are "hidden" in the directories bin/configs/other/openapi3/kotlin* and bin/configs/other/kotlin*.

@wing328 Maybe to avoid this problem of the samples server and openapi3 getting outdated, they should be moved to bin/configs/*?

[jimschubert]

I was going to ask about the DateTime default from java.time.LocalDateTime to java.time.OffsetDateTime, but then I thought about it more and it's really the only type that fully supports the spec's date-time requirement to match rfc3339.

We'll need to consider this a bug fix in that case. Folks may need to pass type-mappings to override for backward compatibility with previously generated code.

[jimschubert]

LGTM