Fix dune version in dune-project

Otherwise we get:

```
- File "test/functoria/e2e/dune", line 18, characters 11-28:
- 18 |   (package functoria-runtime))
-                 ^^^^^^^^^^^^^^^^^
- Error: Dependency on an installed package requires at least (lang dune 2.9)
- (cd _build/default/test/functoria && ./test.exe)
```

dune-project

@@ -1,3 +1,3 @@
-(lang dune 2.8)
+(lang dune 2.9)
 (name mirage)
 (cram enable)