[Fleet] Document required version increase when updating elasticsearch index mappings

[juliaElastic]

Fleet system index mappings are stored in elasticsearch repo. When making mapping changes, the version has to be incremented to apply correctly.

• Document the step to increase version when changing mappings
• Add tests to elasticsearch repo to verify that the version update is done
  • If possible add tests that would fail if mappings are updated without increasing the version

Versioning was introduced here: https://github.com/elastic/elasticsearch/pull/97934/files#diff-2ba90a8ade904611fce0a472e54c4ed346c40bc3ec5fd8481b995c0f8247d987

Pr where version is increased: elastic/elasticsearch#119087

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[juliaElastic]

Seems this documentation issue is already created in https://github.com/elastic/ingest-dev/issues/4866