Fix the PositionModifyRequest object and the "Modify Guild Channel Positions" request

[Azn9]

Expected Behavior:

1. A channel may be moved to another category without having to set its position (it is actually required)
2. The RestGuild#modifyChannelPositions method should return a Mono<Void> as the request returns a 204 No content code.

discord/discord-api-docs#6301 & https://discord.com/developers/docs/resources/guild#modify-guild-channel-positions

Also, the PositionModifyRequest is used for both Role and Channel modification, resolving this issue will need to create a new object specifically for the roles