Adapt `classical` to new semantics

[gebner]

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We've recently changed classical in mathlib to so that it no longer overrides the existing decidable instances. mathlib4 should be adapterd. https://github.com/leanprover-community/mathlib/pull/14122

import Mathlib.Tactic.Basic

example : (by classical; exact decide (0 = 1) : Bool) = decide (0 = 1) := rfl -- this should work