Multi slime readers, Update world version to 1.17, Datafixer, Remove downgrade()

[Owen1212055]

• This correctly changes the world version so it is actually 1.17, as this was forgotten.
• Adds a data fixer to convert cauldrons and dirt paths.
• Splits up slime formats into their own readers, as this will allow modifications to be made without fearing of breaking any previous versions.
• Changed ids for world datafixers, it seems that the ids were totally wrong? My 0x06 (1.16) world was trying to have the 1.16 datafixer run... which isn't really right.
• Removes downgrade() (legacy code)

Splitting up the slime format readers will make it easier when support for custom world heights is added. As this may have special data that needs to be stored or changed.

[Paul19988]

Changes as per requested in discord.