Fix Logger spec for functions working with backends (#8117)
Fix Logger spec for functions working with backends (#8117)

`Logger.add_backend/1,2` and `Logger.remove_backend/1,2` are specced
to only allow atoms as backends, while `Logger.configure_backend/2`
allows backend to be of the more general `backend` type. But both
`add_backend` and `remove_backend` work fine with the more generic
`backend` type, so the specs should be extended to cover that type.

The specs were introduced with c24a13ebf8, while the spec for
`configure_backends` was introduced in aeefd59b15 (with a small fix in
b98421fe51).
ggcampinho/elixir,elixir-lang/elixir,pedrosnk/elixir,kimshrier/elixir,michalmuskala/elixir,ggcampinho/elixir,lexmag/elixir,kimshrier/elixir,kelvinst/elixir,pedrosnk/elixir,joshprice/elixir,kelvinst/elixir,lexmag/elixir