Deprecate -XTypeInType

According to the accepted proposal Embrace (Type :: Type)

Two releases after this proposal is implemented, deprecate -XTypeInType.

The proposal has been implemented in GHC 8.6. We can now deprecate -XTypeInType. The deprecation message should mention that it is replaced by DataKinds + PolyKinds.

Edited Aug 31, 2021 by Krzysztof Gogolewski

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