Use graphql-codegen with legacy mode

Because we are using an older version react-query before it changed to TanStack, graphql-codegen references the new react-query package path, which does not exist in this project. By setting `legacyMode: true` we can instruct codegen to use the old react-query path as documented here:
https://the-guild.dev/graphql/codegen/plugins/typescript/typescript-react-query#legacymode

dbc-gateway.codegen.yml

@@ -11,6 +11,7 @@ generates:
         func: "../graphql-fetcher#fetcher"
       namingConvention: "./scripts/dbc-gateway.codegen.naming"
       defaultScalarType: unknown
+      legacyMode: true
     plugins:
       - "typescript"
       - "typescript-operations"