[dot-kibana-split] Make indices.getMapping() resilient to intermitent failures

[gsoldevila]

When the migrator logic kicks in at startup, it checks the indexTypesMap stored in the main SO index (aka .kibana).
It does so by using esClient.indices.getMapping() method.

Should this method fail due to temporary ES failures (i.e. retriable errors), we want this logic to retry forever, rather than failing and bringing the whole Kibana down.

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: 3b58e44

Metrics [docs]

Unknown metric groups

ESLint disabled line counts

id	before	after	diff
enterpriseSearch	17	19	+2
securitySolution	396	399	+3
total			+5

Total ESLint disabled count

id	before	after	diff
enterpriseSearch	18	20	+2
securitySolution	476	479	+3
total			+5

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@elasticmachine merge upstream

[TinaHeiligers]

@gsoldevila We need to tell QA how to test this change and others that are follow ups to the 'main' PR where we split the indices.
They're going to see a bunch of PR's related to migrations and won't have enough background info to see what they should focus on.
Either add a "How to test this" here or in a detailed document to share with them.
Code changes LGTM.

[gsoldevila]

Either add a "How to test this" here or in a detailed document to share with them.

That's a good point. I'll liaise with Rudolf to help me shape an appropriate document for QA.