deps: limit flufl-lock to <8 (#9681)

Looks like flufl-lock switched to pdm in 8.0 which triggered a

```
pkg_resources.DistributionNotFound: The 'flufl.lock>=5' distribution was not found and is required by dvc
```

error in multiple scenarious. Can't put my finger on anything specific
in flufl-lock yet, so will just limit its version for now.

Fixes https://github.com/iterative/dvc/issues/9654

pyproject.toml

@@ -40,7 +40,8 @@ dependencies = [
     "dvc-studio-client>=0.9.2,<1",
     "dvc-task>=0.3.0,<1",
     "flatten_dict<1,>=0.4.1",
-    "flufl.lock>=5",
+    # https://github.com/iterative/dvc/issues/9654
+    "flufl.lock>=5,<8",
     "funcy>=1.14",
     "grandalf<1,>=0.7",
     "hydra-core>=1.1",