Target inference for stanzas rule in long form should not be allowed if dune lang < 2.0.0

[voodoos]

Reproduction:

• A dune-project file with lang dune < 2.0.0
• A dune file with a stanza rule in long form without explicit targets:

(rule
  (action (copy# some_file.ml.%s some_file.ml)))

(note that this is indeed the long form, short would be (rule (copy# some_file.ml.%s some_file.ml)))

Expected Behavior

Latest Dune should refuse to build the project because target inference for the long forms of the stanza rule was introduced in version 2.0.0 of the dune language (#2494, fixes #2469).

Actual Behavior

Latest Dune builds the project without complaining.

Specifications

• Version of dune: 2.7.0 (but probably all versions >= 2.0.0 are affected)
• Version of ocaml: 4.11.0
• Operating system: MacOS Catalina

Other

See ocaml/merlin#1157

[emillon]

Duplicate of #3230