Use Gitlab::Search::Client instead of Elastic::Helper.default.client in EE

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What does this MR do and why?

Use Gitlab::Search::Client instead of Elastic::Helper.default.client in EE

• Replace direct client usage with Gitlab::Search::Client
• Add es_client accessor for specs per note in issue
• Update spec helper docs

References

#480320 (closed)

How to set up and validate locally

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Related to #480320 (closed)