Improve performance of Promo Code Batch Builder

As pointed out in
https://github.com/solidusio/solidus/pull/5383/files#r1391519251, the
new promotion code batch builder had worse performance characteristics
than the one in the legacy promotion system. This gets the
characteristics back to those of the legacy promotion system.

Time spent and memory usage still go up significantly and linearly with
the number of batches. I ran some specs with the following code:

```rb
  context "with a very large number of promotion codes" do
    let(:number_of_codes) { 10000 }

    it "creates the correct number of codes" do
      puts Benchmark.measure { subject.build_promotion_codes }
      expect(promotion.codes.size).to eq(number_of_codes)
    end
  end
```

With a 1000 codes, I measured:

```
Randomized with seed 49097
  1.036322   0.028977   1.065299 (  1.074396)
```

With 10000 codes, I measured:

```
Randomized with seed 33250
  9.606364   0.278920   9.885284 (  9.978242)
```

Memory usage went up linearly as well.

promotions/app/models/solidus_promotions/promotion_code/batch_builder.rb

@@ -43,15 +43,16 @@ def generate_random_codes
           new_codes = Array.new(max_codes_to_generate) { generate_random_code }.uniq
           codes_for_current_batch = get_unique_codes(new_codes)
 
-          codes_for_current_batch = codes_for_current_batch.map do |value|
+          codes_for_current_batch.filter! do |value|
             SolidusPromotions::PromotionCode.create!(
               value: value,
               promotion: promotion,
               promotion_code_batch: promotion_code_batch
             )
           rescue ActiveRecord::RecordInvalid
-            nil
-          end.compact
+            false
+          end
+
           created_codes += codes_for_current_batch.size
         end
       end