This includes a bugfix to the cty/msgpack package to ensure correct
decoding of unknown and null values.
This also includes updates to cty's dependencies.
This puts us on a version that has grpc protocol support. Although we're
not actually using that yet, the plugin has handshake changed slightly to
allow plugins to declare whether they use the old or new protocols, and
so this upgrade allows us to support plugins that were built against
newer versions of go-plugin that include this extra field in the
handshake.
This fixes#15756.
