Migrate Redis hll values to integers for 'i_code_review_create_mr' keys

What does this MR do and why?

Adds a the a correct version of the migration that was removed in !140587 (merged)

The only difference is that scan_each is called with count: 10_000 which overrides the default value of 10. This change means that the migration will run in a few minutes

For context see #425302 (comment 1727550894)

MR acceptance checklist

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Before	After

How to set up and validate locally

1. Add some RedisHLL keys

   gdk redis-cli pfadd '{hll_counters}_i_code_review_create_mr-2024-03' 1 2 3
   gdk redis-cli pfadd '{hll_counters}_i_code_review_create_mr-2024-02' 4 5 6 
   gdk redis-cli pfadd '{hll_counters}_i_code_review_create_mr-2024-01' 7 8
   gdk redis-cli pfadd '{hll_counters}_i_code_review_create_mr-2023-50' 9 10
   gdk redis-cli pfadd '{hll_counters}_i_code_review_create_mr-2023-49' 11

2. Get the values:

   gdk redis-cli pfcount '{hll_counters}_i_code_review_create_mr-2024-03'
   gdk redis-cli pfcount '{hll_counters}_i_code_review_create_mr-2024-02'
   gdk redis-cli pfcount '{hll_counters}_i_code_review_create_mr-2024-01'
   gdk redis-cli pfcount '{hll_counters}_i_code_review_create_mr-2023-50'
   gdk redis-cli pfcount '{hll_counters}_i_code_review_create_mr-2023-49'

3. Get the current {event_counters} values in case they were already used:

   gdk redis-cli get {event_counters}_i_code_review_user_create_mr-2024-03
   gdk redis-cli get {event_counters}_i_code_review_user_create_mr-2024-02
   gdk redis-cli get {event_counters}_i_code_review_user_create_mr-2024-01
   gdk redis-cli get {event_counters}_i_code_review_user_create_mr-2023-50
   gdk redis-cli get {event_counters}_i_code_review_user_create_mr-2023-49

4. Run the migration bundle exec rake db:migrate
5. Verify that each of the {event_counters} values are the sum of the values you got in step 2) and 3)

Related to #425302 (closed)