[kotlin-client] add ignoreUnknownKeys for kotlinx serialization

[kassim]

fixes #13013

initialises the Json instance for kotlinx serialization with ignoreUnknownKeys = true so that deserialisation doesn't fail when new keys are added to the API

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[4brunu]

@kassim Hey, thanks for opening this PR.
I have created #12930 that among other things it also fixes this.
Could you please take a look at it to see if it fixes this issue?
Thanks

[kassim]

@4brunu it does fix it – thank you!