Bump links to CF-1.11 (#519)

Author: dcherian

cf_xarray/accessor.py

@@ -2274,8 +2274,8 @@ def formula_terms(self) -> dict[Hashable, dict[str, str]]:  # numpydoc ignore=SS
         References
         ----------
         Please refer to the CF conventions document :
-          1. http://cfconventions.org/Data/cf-conventions/cf-conventions-1.8/cf-conventions.html#parametric-vertical-coordinate
-          2. http://cfconventions.org/Data/cf-conventions/cf-conventions-1.8/cf-conventions.html#parametric-v-coord.
+          1. http://cfconventions.org/Data/cf-conventions/cf-conventions-1.11/cf-conventions.html#parametric-vertical-coordinate
+          2. http://cfconventions.org/Data/cf-conventions/cf-conventions-1.11/cf-conventions.html#parametric-v-coord.
 
         Examples
         --------
@@ -2551,7 +2551,7 @@ def bounds_to_vertices(
 
         References
         ----------
-        Please refer to the CF conventions document : http://cfconventions.org/Data/cf-conventions/cf-conventions-1.8/cf-conventions.html#cell-boundaries.
+        Please refer to the CF conventions document : http://cfconventions.org/Data/cf-conventions/cf-conventions-1.11/cf-conventions.html#cell-boundaries.
         """
         if keys is None:
             coords = tuple(self.keys())
@@ -2760,8 +2760,8 @@ def formula_terms(self) -> dict[str, str]:  # numpydoc ignore=SS06
         References
         ----------
         Please refer to the CF conventions document :
-          1. http://cfconventions.org/Data/cf-conventions/cf-conventions-1.8/cf-conventions.html#parametric-vertical-coordinate
-          2. http://cfconventions.org/Data/cf-conventions/cf-conventions-1.8/cf-conventions.html#parametric-v-coord.
+          1. http://cfconventions.org/Data/cf-conventions/cf-conventions-1.11/cf-conventions.html#parametric-vertical-coordinate
+          2. http://cfconventions.org/Data/cf-conventions/cf-conventions-1.11/cf-conventions.html#parametric-v-coord.
 
         Examples
         --------