Account for Rails 7 rounding in time spec

Storing a timestamp to the database has less accuracy than a Ruby Time
object. So `updated_at` changes after being written and loaded from the
database. Rails 7 accounts for that by rounding it in the model already
before it's written to the database. That made one spec fail.

spec/models/spree/product_spec.rb

@@ -309,7 +309,7 @@ module Spree
       it "defaults available_on to now" do
         Timecop.freeze do
           product = Product.new
-          expect(product.available_on).to eq(Time.zone.now)
+          expect(product.available_on).to be_within(0.000001).of(Time.zone.now)
         end
       end