Fix panic message to show correct unhandled type rather than the parent type

[rm-hull]

I was trying to model a block that had map[string][]string as it's schema, but the code was panicking with:

Unknown validation type: 6

Debugging, and it became clear that non-primitive types are not supported in the switch, but I was puzzled as to why it was reporting that a TypeMap was not a valid map value.

The switch is against valueType, so it makes sense (to me at least) that the panic should report that as the unknown validation type.

With this change, the code would report:

Unknown validation type: 7

Where 7 equates to TypeSet (aka. the string slice value of the map)

[hashicorp-cla]

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[github-actions[bot]]

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