[sledge] Build: Require dune >= 2.7
[sledge] Build: Require dune >= 2.7

Summary:
The dune build info embedded into executables is broken with dune <
2.7 and an flambda compiler.

Reviewed By: ngorogiannis

Differential Revision: D23459510

fbshipit-source-id: 18b3e3e15

facebook/infer,jvillard/infer,facebook/infer,jvillard/infer,facebook/infer,facebook/infer,facebook/infer,facebook/infer,jvillard/infer,facebook/infer,jvillard/infer,jvillard/infer,jvillard/infer,jvillard/infer