Fix CacheControl setter type stubs

mypy doesn't use the type of setters as of 1.9.0 (see python/mypy#3004),
but I think it's still good to have these be accurate (maybe the other
type checkers work better here).

src/werkzeug/datastructures/cache_control.pyi

@@ -27,7 +27,7 @@ class _CacheControl(
     @property
     def no_cache(self) -> str | None: ...
     @no_cache.setter
-    def no_cache(self, value: bool | None) -> None: ...
+    def no_cache(self, value: Literal[True] | str | None) -> None: ...
     @no_cache.deleter
     def no_cache(self) -> None: ...
     @property
@@ -83,7 +83,7 @@ class ResponseCacheControl(_CacheControl):
     @property
     def private(self) -> str | None: ...
     @private.setter
-    def private(self, value: bool | None) -> None: ...
+    def private(self, value: Literal[True] | str | None) -> None: ...
     @private.deleter
     def private(self) -> None: ...
     @property