[dune] make lang version coherent

doc/developer/tutorial/viewcfg/v5-state/dune-project

doc/developer/tutorial/viewcfg/v6-state-clear/dune-project

doc/userman/user-plugins.tex

src/libraries/qed/dune-project

src/plugins/alias/dune-project

src/plugins/aorai/dune-project

src/plugins/api-generator/dune-project

src/plugins/callgraph/dune-project

src/plugins/constant_propagation/dune-project

src/plugins/dive/dune-project

src/plugins/e-acsl/dune-project

src/plugins/eva/dune-project

src/plugins/from/dune-project

src/plugins/impact/dune-project

src/plugins/inout/dune-project

src/plugins/instantiate/dune-project

src/plugins/loop_analysis/dune-project

src/plugins/markdown-report/dune-project

src/plugins/metrics/dune-project

src/plugins/nonterm/dune-project