Use java.time.Instant to represent DateTime instead of OffsetDateTime

just add to the configuration of openapi-generator-maven-plugin

<typeMappings>
     <typeMapping>OffsetDateTime=Instant</typeMapping>
</typeMappings>
<importMappings>                                
     <importMapping>java.time.OffsetDateTime=java.time.Instant</importMapping>
</importMappings>