Fix max number of slices

What does this MR do and why?

It turns out that a few of our migrations weren't handling indices with 1 shard correctly. This MR fixes it.

Elasticsearch::Transport::Transport::Errors::BadRequest: [400] {"error":{"root_cause":[{"type":"x_content_parse_exception","reason":"[1:17] [slice] failed to parse field [max]"}],"type":"x_content_parse_exception","reason":"[1:351] [reindex] failed to parse field [source]","caused_by":{"type":"x_content_parse_exception","reason":"[1:17] [slice] failed to parse field [max]","caused_by":{"type":"illegal_argument_exception","reason":"max must be greater than 1"}}},"status":400
}

#420895 (closed)

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Before	After

How to set up and validate locally

It's a bit tricky to test, but the closest test, which will break your project index, is

1. Check out master
2. Run curl -XDELETE 'http://localhost:9200/gitlab-development-migrations/_doc/20230530500000'
3. Ensure that Elastic::DataMigrationService[20230530500000].send(:migration).migrate doesn't raise an exception
4. Update the number of shards for your indices to 1
5. Reindex your instance
6. curl -XDELETE 'http://localhost:9200/gitlab-development-migrations/_doc/20230530500000'
7. Ensure that Elastic::DataMigrationService[20230530500000].send(:migration).migrate raises an exception
8. Checkout the branch of this MR
9. Run curl -XDELETE 'http://localhost:9200/gitlab-development-migrations/_doc/20230530500000'
10. Ensure that Elastic::DataMigrationService[20230530500000].send(:migration).migrate works correctly now

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.