List comprehensions are also a great choice for things like this:

list = [
  %{id: 7, name: "A", count: 1},
  %{id: 8, name: "B", count: 1},
  %{id: 9, name: "C", count: 0}
]

for elem <- list do
  for {key, val} <- elem, into: %{} do
    {to_string(key), to_string(val)}
  end
end

Another way to do this would be using the Jason library:

iex> data = %{:some => %{:nested => :data}}
%{some: %{nested: :data}}

iex> data |> Jason.encode!() |> Jason.decode!()
%{"some" => %{"nested" => "data"}}

@Siel gave you already an answer that works, though Map.new/2 might be more performant than Enum.map/2 |> Enum.into/2.

Also be aware that this works only for a single level of maps. If your maps are more complicated or nested, then you need to do a lot more of the heavy lifting and do some explicit recursion.