Remove: support ElasticSearch 5.x & 6.x

[tico88612]

Which problem is this PR solving?

• Relate [Deprecation] Drop support for Elasticsearch v5 / v6 jaeger#5439

Description of the changes

• Remove ElasticSearch 5.x & 6.x documentation

How was this change tested?

• Manual

Checklist

• I have read https://github.com/jaegertracing/jaeger/blob/master/CONTRIBUTING_GUIDELINES.md
• I have signed all commits
• I have added unit tests for the new functionality
• I have run lint and test steps successfully
  • for jaeger: make lint test
  • for jaeger-ui: yarn lint and yarn test

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[yurishkuro]

$ grx 5.x content/docs/next-release
content/docs/next-release/deployment.md:375:* Supported ES versions: 5.x, 6.x, 7.x, 8.x (since Jaeger v1.52.0)
content/docs/next-release/deployment.md:538:until indices created by ES 5.x are removed or explicitly reindexed.
content/docs/next-release/features.md:23:* It natively supports popular open source NoSQL databases as trace storage backends: Cassandra 3.4+, Elasticsearch 5.x/6.x/7.x, and OpenSearch 1.0+.
content/docs/next-release/_index.md:46:    * [Elasticsearch 5.x, 6.x, 7.x](./deployment/#elasticsearch)

[yurishkuro]

Thanks