Remove support for Elasticsearch v5/v6 (#5448)

## Which problem is this PR solving?
- Resolves second point in the checklist of #5439 

## Description of the changes
- Removed support for Elasticsearch v5/v6 in mapping.go and
mapping_test.go

## How was this change tested?
- 

## 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`

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Signed-off-by: FlamingSaint <raghuramkannan400@gmail.com>