Bump Hammer deps to 7

Is it possible to bump version of hammer to 7.x? Thank you!

```
Because hammer_plug >= 2.0.0 depends on hammer ~> 6.0 and your app depends on hammer ~> 7.1, hammer_plug >= 2.0.0 is forbidden.
So, because your app depends on hammer_plug ~> 3.2, version solving failed.
** (Mix) Hex dependency resolution failed
```