Change mapgen order to ores > dungeons > decorations (#7656)

Previously dungeons > decorations > ores.
Decorations depend on the node they are placed on, and ore types are
increasingly able to place large continuous volumes of nodes, such as
strata. Decorations are increasingly being used underground.

The new order avoids a node being changed by ore generation after a
decoration has been placed on it.
The new order also avoids ores appearing in the walls of a dungeon if
the wall nodes match the local stone node.

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