Remove unsupported :same_site option value [ci-skip]

Rails 7.0 does not support opting out of the `SameSite` attribute by
specifying `same_site: nil`.  (However, #45501 will add support for this
in the next Rails release.)

actionpack/lib/action_dispatch/middleware/cookies.rb

@@ -178,8 +178,7 @@ def use_cookies_with_metadata
   #   only HTTP. Defaults to +false+.
   # * <tt>:same_site</tt> - The value of the +SameSite+ cookie attribute, which
   #   determines how this cookie should be restricted in cross-site contexts.
-  #   Possible values are +nil+, +:none+, +:lax+, and +:strict+. Defaults to
-  #   +:lax+.
+  #   Possible values are +:none+, +:lax+, and +:strict+. Defaults to +:lax+.
   class Cookies
     HTTP_HEADER   = "Set-Cookie"
     GENERATOR_KEY = "action_dispatch.key_generator"