Add a migration to reindex already existing group wikis

What does this MR do and why?

We have already implemented the indexing of the GroupWikis but we still need a migration to reindex all the GroupWikis which were created or updated before the implementation of the GroupWikis. This migration will reindex all the GroupWikis which are enabled to use elasticsearch

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Before	After

How to set up and validate locally

Make sure Elasticsearch is enabled

1. Create a group wiki in the GDK

2. To simulate the scenario of not having group wikis in Elasticsearch, you have to delete all the group wikis from the elastic

curl -H 'Content-type: application/json' -XPOST 'http://localhost:9200/gitlab-development-wikis/_delete_by_query -d'
{
  "body": {
    "query": {
      "regexp": {
        "rid": "wiki_group_[0-9].*"
      }
    }
  }
}'

3. Open the Rails console

bundle exec rails c

4. Run the following command

require_relative 'ee/elastic/migrate/20230625500000_backfill_existing_group_wiki.rb' 
BackfillExistingGroupWiki.new(20230625500000).migrate

3. Check group wikis on ES

curl -H 'Content-type: application/json' -XGET 'http://localhost:9200/gitlab-development-wikis/_search -d'
{
  "body": {
    "query": {
      "regexp": {
        "rid": "wiki_group_[0-9].*"
      }
    }
  }
}'

It should not be empty

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Approximate time to completion

~ 24 hours

No. of iterations needed

56191/200 =~ 280

[1] pry(main)> number_of_groups = GroupWikiRepository.count
=> 56191
[2] pry(main)> throttle_delay = 5.minute
=> 5 minutes
[3] pry(main)> batch_size = 200
=> 200
[4] pry(main)> ((number_of_groups / batch_size) * throttle_delay) / 1.hour
=> 23

Related to #336100 (closed)