Weaken unavailable setter to deprecated

Swift <6.3 had a soundness hole that allowed these writes to happen
_via_, _e.g._, dynamic member lookup. This is quite disruptive to
existing code bases, so let's warn instead, clearly noting a potential
data race, and allow folks to continue building.

Author: stephencelis

Sources/Sharing/Shared.swift

@@ -106,8 +106,10 @@ public struct Shared<Value> {
       }
       @available(
         *,
-        unavailable,
-        message: "Use '$shared.withLock' to modify a shared value with exclusive access."
+        deprecated,
+        message: """
+          Use '$shared.withLock' to modify a shared value with exclusive access; when constructing a SwiftUI binding, use 'Binding($shared)'
+          """
       )
       nonmutating set {
         withLock { $0 = newValue }