Header returns a middleware handler that sets the `Server` header to the name of the application and the (truncated) build revision.

Revision is the global build revision from source control as used by the ServerHeader middleware, to be set using compiler ldflags like: `-ldflags="-X github.com/cuvva/cuvva-public-go/lib/version.Revision=$(git rev-parse HEAD)"`.

Truncated is up to 7 characters of Revision - so git hashes look as they are typically shown in GUIs, on GitHub, etc.