[API] Bump API version (skypilot-org#5866)

Bump API version

Author: lucamanolache

sky/server/constants.py

@@ -7,7 +7,7 @@
 # API server version, whenever there is a change in API server that requires a
 # restart of the local API server or error out when the client does not match
 # the server version.
-API_VERSION = '8'
+API_VERSION = '9'
 
 # Prefix for API request names.
 REQUEST_NAME_PREFIX = 'sky.'