Downgrading breaks chat file compatibility - fix before v3.4.0 release

[cebtenzzre]

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We serialize CHAT_FORMAT_VERSION, but when we deserialize we do not validate the version against this, we only compare it with 1. Version 10 chats in the data folder of a version of GPT4All capable of reading only version 9 chats causes a hang (or possibly a crash in some cases).