[3.0] Fix force experimental gameplay

[KCodeYT]

StartGamePacket#forceExperimentalGameplay is not an optional boolean, so if you send another boolean after the first one that already said that force experimental gameplay was enabled, the bytes afterwards are corrupted and this is resulting in the client closing the connection.

Tested with: https://github.com/CloudburstMC/Server/tree/refactor/items

startGamePacket.setForceExperimentalGameplay(OptionalBoolean.of(true));

Changed to:

startGamePacket.setForceExperimentalGameplay(true);

[dktapps]

This does not look correct, it is and always has been an optional to the best of my knowledge. You likely have some other encoding issue.