Remove: support ElasticSearch 5.x & 6.x

[tico88612]

Which problem is this PR solving?

• Relate https://github.com/jaegertracing/jaeger/issues/5439

Description of the changes

• Remove ElasticSearch 5.x & 6.x documentation

How was this change tested?

• Manual

Checklist

• [x] I have read https://github.com/jaegertracing/jaeger/blob/master/CONTRIBUTING_GUIDELINES.md
• [x] 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

[netlify[bot]]

✅ Deploy Preview for romantic-neumann-1959d7 ready!

Name	Link
🔨 Latest commit	83d01520fd29588bf088b2070c9d88e14b75f1cd
🔍 Latest deploy log	https://app.netlify.com/sites/romantic-neumann-1959d7/deploys/6640312ec51ac30008ec2151
😎 Deploy Preview	https://deploy-preview-703--romantic-neumann-1959d7.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.