Fix for determining write index

What does this MR do and why?

Related to #428070 (closed)

Found while working on implementing indexing all projects in: !134456 (merged)

This was causing spec failures when trying to get a document count from Elasticsearch for a specific index.

• refactors logic which determines which index is the write index
• adds a spec for the method changed

Screenshots or screen recordings

N/A - backend change only

How to set up and validate locally

1. setup gdk for elasticsearch
2. checkout master branch
3. create 2 test indexes, using the same alias, with one set to the write index

curl --request PUT \
  --url http://localhost:9200/test-index-2 \
  --header 'Content-Type: application/json' \
  --data '{
	"aliases": {
		"test_alias": {
			"is_write_index": false
		}
	}
}'

curl --request PUT \
  --url http://localhost:9200/test-index-1 \
  --header 'Content-Type: application/json' \
  --data '{
	"aliases": {
		"test_alias": {
			"is_write_index": true
		}
	}
}'

4. in rails console, run the method to find each index. only one should be the write index <-- this is the bug

> elastic_helper = Gitlab::Elastic::Helper.default
> elastic_helper.target_index_names(target: 'test_alias')
=> {"test-index-1"=>true, "test-index-2"=>true}

5. checkout this (428070-fix-for-determining-write-index) branch
6. run the same method in rails console

> elastic_helper = Gitlab::Elastic::Helper.default
> elastic_helper.target_index_names(target: 'test_alias')
=> {"test-index-1"=>true, "test-index-2"=>false}

MR acceptance checklist

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