Update to fmt >= 0.6.3.0

Clarification and motivation

So far, we've been deriving generic instances of Buildable like this:

data Bar = Bar { x :: Bool }
  deriving stock Generic

instance Buildable Bar where
  build = genericF

With the latest fmt update, we can now simplify things a bit using DerivingVia:

data Bar = Bar { x :: Bool }
  deriving stock Generic
  deriving Buildable via GenericBuildable Bar

We should also make sure the cabal/package.yaml files contain the correct lower bound on the fmt dependency.

UPDATE: fmt now depends on formatting >= 7.x, but LTS 17 contains only formatting 6.x. We should take the opportunity to update to LTS 18.

Acceptance criteria

• We've updated to fmt >= 0.6.3.0
• All generic instances now use DerivingVia
• package.yaml has the right lower bound constraint