OfferMessage の multistream を削除する

[zztkm]

リリース前の項目調整のため、CHANGES は書いていません。

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This pull request includes changes to the OfferMessage class and its documentation. The most important changes involve updating the OfferMessage class to remove the multistream field and updating the CHANGES.md file accordingly.

Documentation updates:

• CHANGES.md: Removed the multistream field from the list of added items in the OfferMessage section.

Code updates:

• sora-android-sdk/src/main/kotlin/jp/shiguredo/sora/sdk/channel/signaling/message/Catalog.kt: Removed the multistream field from the OfferMessage data class.

[miosakuma]

対応ありがとうございます、確認しました。