Previously, we would output the version we were parsing which led to
nonsensical error messages like:

```
File "dune-project", line 1, characters 11-18:
  1 | (lang dune 123.123)
                 ^^^^^^^
  Error: Version 123.123 of the dune language is not supported.
  Supported versions of this extension in version 123.123 of the dune language:
  - 1.0 to 1.12
  - 2.0 to 2.9
  - 3.0 to 3.20
```

We fix this by passing the current supported version of dune lang
instead of the one we just parsed, allowing the error message to
beceome:

```
  File "dune-project", line 1, characters 11-18:
  1 | (lang dune 123.123)
                 ^^^^^^^
  Error: Version 123.123 of the dune language is not supported.
  Supported versions of this extension in version 3.20 of the dune language:
  - 1.0 to 1.12
  - 2.0 to 2.9
  - 3.0 to 3.20
```

The error message is still a bit nonsensical due to us mentioning
extensions here, but that will be left as a future improvement.

Signed-off-by: Ali Caglayan <alizter@gmail.com>

When we display an error for an outdated dune version which can happen
when `dune lang` appears to be unsupported by the current version of
dune, we used to mention an "extension". This was confusing to users who
didn't know about language extensions or that dune lang was an extension
under the hood.

We improve the error message by using some of the information already
provided by dune_lang/syntax.ml.

Signed-off-by: Ali Caglayan <alizter@gmail.com>