[BUG][Kotlin] Fix wrong type- and importMapping for kotlinx.date (#17597)

[ch4rl3x]

Closes: #17597

@jimschubert, @dr4ke616, @karismann, @Zomzog, @andrewemery, @4brunu, @yutaka0m, @stefankoppier

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[4brunu]

Looks good to me 👍

[wing328]

@ch4rl3x thanks for the PR. When you've time, can you please PM me via Slack?

https://join.slack.com/t/openapi-generator/shared_invite/zt-12jxxd7p2-XUeQM~4pzsU9x~eGLQqX2g