Forbid using elasticsearch.username: elastic in production

[jportner]

Resolves #51101.

This was already forbidden when running Kibana from source, now it is also forbidden when running Kibana from the distribution (i.e., in production).

If you configure Kibana with elasticsearch.username: elastic, it will crash with the following error:

[2022-01-11T17:18:15.527-05:00][FATAL][root] Error: [config validation of [elasticsearch].username]: value of "elastic" is forbidden. This is a superuser account that cannot write to system indices that Kibana needs to function. Use a service account token instead. Learn more: https://www.elastic.co/guide/en/elasticsearch/reference/8.0/service-accounts.html
at ensureValidConfiguration (/Users/joe/GitHub/kibana-2/src/core/server/config/ensure_valid_configuration.ts:24:11)
at Server.preboot (/Users/joe/GitHub/kibana-2/src/core/server/server.ts:140:5)
at Root.preboot (/Users/joe/GitHub/kibana-2/src/core/server/root/index.ts:42:14)
at bootstrap (/Users/joe/GitHub/kibana-2/src/core/server/bootstrap.ts:84:25)
at Command. (/Users/joe/GitHub/kibana-2/src/cli/serve/serve.js:244:5)

7.17 companion PR: #122717

---

8.0 release note: Kibana can no longer use the elastic superuser to authenticate to Elasticsearch (configuring elasticsearch.username: elastic is forbidden).

[kibana-ci]

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✅ unchanged

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• 💔 Build #16849 failed adaad75

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Status	Branch	Result
✅	8.0

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