Use correct fmt version in dune-project

[craigfe]

As of Dune 2.5.0, warnings are emitted if the (using lang <foo>) stanza is using a version higher than is supported. This gives us the following warning in CI:

File "dune-project", line 3, characters 11-14:
3 | (using fmt 1.2)
               ^^^
Warning: Version 1.2 of integration with automatic formatters is not
supported until version 1.11 of the dune language.
Supported versions of this extension in version 1.8 of the dune language:
- 1.0 to 1.1

[samoht]

I guess an other option is to upgrade the version of the dune-project instead? Not sure what's the difference between fmt 1.1 and fmt 1.2.

[pascutto]

1.2 enables support for dialects, which we don't use here. I think it's fine to downgrade thefmt version, everything will upgrade to dune.2.0.0+ with #991 eventually anyways.