Support Elixir 1.18 JSON module

[maxsalven]

Using json_codec: JSON doesn't work as this library calls encode!/2 with a Jason specific second argument (a keyword list) which isn't compatible with JSON.encode!/2.

absinthe_plug/lib/absinthe/plug.ex

Line 607 in 5de21da

|> send_resp(status, mod.encode!(body, opts))

See elixir-lang/elixir#14317

[maxsalven]

A related note, should we be calling encode_to_iodata!/2 instead of encode!/2 here?