[BUG][kotlin][client] Wrong date-time object by default

[4brunu]

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Description

By default, the Kotlin client uses LocalDateTime to represent the date-time received from the server.
This is because because LocalDateTime doesn't have a notion of timezone.

openapi-generator version

4.2.2

Steps to reproduce

Related issues/PRs

Suggest a fix

It would be better to represent the date time received by the server in a OffsetDateTime which contains the timezone information.
The Java generator and the KotlinSpringServer also use the OffsetDateTime.

openapi-generator/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractJavaCodegen.java

Lines 471 to 476 in fb0479a

	additionalProperties.put("threetenbp", "true");
	additionalProperties.put("jsr310", "true");
	typeMapping.put("date", "LocalDate");
	typeMapping.put("DateTime", "OffsetDateTime");
	importMapping.put("LocalDate", "org.threeten.bp.LocalDate");
	importMapping.put("OffsetDateTime", "org.threeten.bp.OffsetDateTime");

openapi-generator/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/KotlinSpringServerCodegen.java

Lines 104 to 107 in fb0479a

	typeMapping.put("date", "java.time.LocalDate");
	typeMapping.put("date-time", "java.time.OffsetDateTime");
	typeMapping.put("Date", "java.time.LocalDate");
	typeMapping.put("DateTime", "java.time.OffsetDateTime");

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