Migrate unless to if ! for Elixir 1.18 compatibility

lib/oauth2/strategy/auth_code.ex

@@ -46,7 +46,7 @@ defmodule OAuth2.Strategy.AuthCode do
   def get_token(client, params, headers) do
     {code, params} = Keyword.pop(params, :code, client.params["code"])
 
-    unless code do
+    if !code do
       raise OAuth2.Error, reason: "Missing required key `code` for `#{inspect(__MODULE__)}`"
     end
 

lib/oauth2/strategy/password.ex

@@ -38,7 +38,7 @@ defmodule OAuth2.Strategy.Password do
     {username, params} = Keyword.pop(params, :username, client.params["username"])
     {password, params} = Keyword.pop(params, :password, client.params["password"])
 
-    unless username && password do
+    if !(username && password) do
       raise OAuth2.Error,
         reason: "Missing required keys `username` and `password` for #{inspect(__MODULE__)}"
     end

lib/oauth2/strategy/refresh.ex

@@ -39,7 +39,7 @@ defmodule OAuth2.Strategy.Refresh do
   def get_token(client, params, headers) do
     {token, params} = Keyword.pop(params, :refresh_token, client.params["refresh_token"])
 
-    unless token do
+    if !token do
       raise OAuth2.Error,
         reason: "Missing required key `refresh_token` for `#{inspect(__MODULE__)}`"
     end