fix: Convert Devise.sign_out_via to String

Since lastest Alchemy main (8.1.0.a) this needs
to be a String and cannot be a Symbol.

Author: tvdeyen

config/initializers/alchemy.rb

@@ -18,5 +18,5 @@
   Alchemy.signup_path = "/admin/signup"
   Alchemy.login_path = "/admin/login"
   Alchemy.logout_path = "/admin/logout"
-  Alchemy.logout_method = Devise.sign_out_via
+  Alchemy.logout_method = Devise.sign_out_via.to_s
 end