Fix dune build @doc-full infrastructure
- Bump root dune-project to (lang dune 3.21) for odoc v3 rules
- Remove (package odoc) from odoc_model_semantics_test library
- Remove (package js_top_worker_rpc_def) to work around directory-targets
bug in dune's doc generation
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>