Fail over to the latest valid sat_version

With the adoption of Satellite stream, the logic for selecting a
sat_version for subscription allocations created by manifester is no
longer valid in all cases. This PR modifies the logic to default to the
most recent valid version if initial processing of the provided
sat_version does not yield a valid version.

manifester/helpers.py

@@ -35,15 +35,7 @@ def process_sat_version(sat_version, valid_sat_versions):
             sat_version = sat_version.split('.')
             sat_version = sat_version[0:2]
             sat_version = ".".join(sat_version)
-        # The conditional below assumes that an invalid sat_version with the Z-stream version removed
-        # is a Y-stream version in development. New Y-stream versions are not available as valid 
-        # sat_version values for the 'POST allocations' endpoint until the corresponding Satellite
-        # versions are generally available. As of Satellite 6.12, decrementing the Y-stream version
-        # by 1 works around this constraint, but the conditional below may need to be modified to
-        # accommodate versioning scheme changes or additional use cases. 
+        # If sat_version is still not valid, default to the latest valid version.
         if sat_version not in valid_sat_versions:
-            sat_version = sat_version.split('.')
-            sat_version[1] = str(int(sat_version[1]) - 1)
-            sat_version = ".".join(sat_version)
-        assert sat_version in valid_sat_versions
-    return sat_version
+            valid_sat_versions.sort(key = lambda i: int(i.split('-')[-1].split('.')[-1]), reverse = True)
+    return valid_sat_versions[0]