[AutoDiff] Update @differentiable attribute API/ABI status. (#28148)

Mark `@differentiable` attribute as:
- API/ABI stable to add. (The `@differentiable` declaration is unaffected.)
- API/ABI breaking to remove.

Author: dan-zheng

include/swift/AST/Attr.def

@@ -506,7 +506,7 @@ DECL_ATTR(_projectedValueProperty, ProjectedValueProperty,
 DECL_ATTR(differentiable, Differentiable,
   OnAccessor | OnConstructor | OnFunc | OnVar | OnSubscript | LongAttribute |
   AllowMultipleAttributes |
-  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  ABIStableToAdd | ABIBreakingToRemove | APIStableToAdd | APIBreakingToRemove,
   90)
 DECL_ATTR(differentiating, Differentiating,
   OnFunc | LongAttribute | AllowMultipleAttributes |