chore: fix Elixir 1.17 warnings

[jswanner]

Resolves the following warnings:

using map.field notation (without parentheses) to invoke function Stripe.CountrySpec.__struct__() is deprecated, you must add parentheses instead: remote.function()

single-quoted strings represent charlists. Use ~c"" if you indeed want a charlist or use "" instead

[yordis]

🚀 💜

[ceolinwill]

@yordis It would be great if you could release a new version with this fix.