Improve typing of Stripe::ApiResource#metadata

Per the docs, values are always strings (there is no nesting). I opted to not change the signature of `#metadata=` because you can technically pass in non-strings to the method and either the values will be stringified (e.g. `1` becomes `"1"`) or the keys ignored (e.g. a key with a `nil` value).

https://docs.stripe.com/metadata

rbi/annotations/stripe.rbi

@@ -12,7 +12,7 @@ class Stripe::APIResource < Stripe::StripeObject
 
   # not all objects, at all times have metadata (deleted customers for instance)
   # @method_missing: from StripeObject
-  sig { returns(T::Hash[T.any(String, Symbol), T.untyped]) }
+  sig { returns(T::Hash[T.any(String, Symbol), String]) }
   def metadata; end
 
   # @method_missing: from StripeObject