Point users to where in their code they should make mods for Dataset.dims (#8534)

* Point users to where in their code they should make mods for Dataset.dims

Its somewhat annoying to get warnings that point to a line within a library where the warning is issued. It really makes it unclear what one needs to change.

This points to the user's access of the `dims` attribute.

* use emit_user_level_warning

Author: hmaarrfk

xarray/core/utils.py

@@ -491,7 +491,7 @@ class FrozenMappingWarningOnValuesAccess(Frozen[K, V]):
     __slots__ = ("mapping",)
 
     def _warn(self) -> None:
-        warnings.warn(
+        emit_user_level_warning(
             "The return type of `Dataset.dims` will be changed to return a set of dimension names in future, "
             "in order to be more consistent with `DataArray.dims`. To access a mapping from dimension names to lengths, "
             "please use `Dataset.sizes`.",