fix: Make project optional when creating container registry

Author: seedspirit

docs/manager/graphql-reference/schema.graphql

@@ -2442,7 +2442,7 @@ type Mutation {
     password: String
 
     """Added in 24.09.0."""
-    project: String!
+    project: String
 
     """Added in 24.09.0."""
     registry_name: String!

docs/manager/graphql-reference/supergraph.graphql

@@ -4552,7 +4552,7 @@ type Mutation
     password: String
 
     """Added in 24.09.0."""
-    project: String!
+    project: String
 
     """Added in 24.09.0."""
     registry_name: String!

src/ai/backend/manager/models/gql_models/container_registry.py

@@ -315,7 +315,7 @@ class Arguments:
         )
         registry_name = graphene.String(required=True, description="Added in 24.09.0.")
         is_global = graphene.Boolean(description="Added in 24.09.0.")
-        project = graphene.String(required=True, description="Added in 24.09.0.")
+        project = graphene.String(description="Added in 24.09.0.")
         username = graphene.String(description="Added in 24.09.0.")
         password = graphene.String(description="Added in 24.09.0.")
         ssl_verify = graphene.Boolean(description="Added in 24.09.0.")