Warn users of the future removal of platform=cray (#43980)

lib/spack/spack/solver/asp.py

@@ -3776,6 +3776,12 @@ class Solver:
     def __init__(self):
         self.driver = PyclingoDriver()
         self.selector = ReusableSpecsSelector(configuration=spack.config.CONFIG)
+        if spack.platforms.host().name == "cray":
+            msg = (
+                "The Cray platform, i.e. 'platform=cray', will be removed in Spack v0.23. "
+                "All Cray machines will be then detected as 'platform=linux'."
+            )
+            warnings.warn(msg)
 
     @staticmethod
     def _check_input_and_extract_concrete_specs(specs):