This upgrades hcl2 library dependency to pick up
https://github.com/hashicorp/hcl2/pull/113 .
Prior to this change, parsing and decoding array attributes containing
invalid errors (e.g. references to unknown variables) are silently
dropped, with `cty.Unknown` being assigned to the bad element. Rather
than showing a type/meaningful error from hcl2, we get a very decrypted
error message from msgpack layer trying to handle `cty.unknown`.
This ensures that we propagate diagnostics correctly and report
meaningful errors to users.
Fixes https://github.com/hashicorp/nomad/issues/5694
Fixes https://github.com/hashicorp/nomad/issues/5680
