Update dependency org.jetbrains.kotlinx:kotlinx-serialization-json to v1.8.1

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
org.jetbrains.kotlinx:kotlinx-serialization-json	1.8.0 -> 1.8.1

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Release Notes

Kotlin/kotlinx.serialization (org.jetbrains.kotlinx:kotlinx-serialization-json)

v1.8.1

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This release updates Kotlin version to 2.1.20, while also providing several important improvements
and bugfixes.

Improvements

• Implemented encoding null in key and value of a map in Protobuf (#​2910)
• Make type argument in JsonTransformingSerializer nullable (#​2911)
• Use SPDX identifier in POMs (#​2936) (thanks to Leon Linhart)
• Add watchosDeviceArm64 to Okio integration module (#​2920) (thanks to Daniel Santiago)
• Update kotlinx-io version to 0.6.0 (#​2933) (thanks to Piotr Krzemiński)

Bugfixes

• Fix incorrect enum coercion during deserialization from JsonElement (#​2962)
• Supply proper equals(), hashCode(), and toString() for SerialDescriptor() wrapper (#​2942)
• Do not encode empty packed collections in protobuf (#​2907)

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