Upgrade github.com/zclconf/go-cty to v1.11.1

In v1.11.0 go-cty dropped all encoding/gob support - refer to commit for details.
Gob support is currently being used by the Datasource RPC server, and possiblity within other locations, for communication with the data source plugin components. Before upgrading
to the latest go-cty version the RPC serialization code must be updated to use a new serialization format or we can look into copying the encoding/gob support into
the Packer SDK if there are no other options.