Drop support for OTP 20

.github/workflows/ci.yml

@@ -42,7 +42,7 @@ jobs:
             elixir: 1.10.4
           - erlang: 21.3
             elixir: 1.9.4
-          - erlang: 20.3.1
+          - erlang: 21.3
             elixir: 1.8.2
     env:
       GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

lib/json/ld/encoder.ex

@@ -35,6 +35,7 @@ defmodule JSON.LD.Encoder do
     end
   end
 
+  # TODO: unless we find a way to allow more optimized encode! versions, remove this since it's never used (see the respective warning)
   @impl RDF.Serialization.Encoder
   @spec encode!(RDF.Data.t(), Options.t() | Enum.t()) :: String.t()
   @dialyzer {:nowarn_function, encode!: 1}