Remove support for Elasticsearch v5/v6

[FlamingSaint]

Which problem is this PR solving?

• Resolves second point in the checklist of [Deprecation] Drop support for Elasticsearch v5 / v6 #5439

Description of the changes

• Removed support for Elasticsearch v5/v6 in mapping.go and mapping_test.go

How was this change tested?

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.48%. Comparing base (5757c56) to head (79a2f82).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5448      +/-   ##
==========================================
+ Coverage   95.47%   95.48%   +0.01%     
==========================================
  Files         331      331              
  Lines       16115    16113       -2     
==========================================
  Hits        15385    15385              
+ Misses        556      555       -1     
+ Partials      174      173       -1     

Flag	Coverage Δ
badger_v1	8.07% <0.00%> (+<0.01%)	⬆️
badger_v2	1.94% <0.00%> (?)
cassandra-3.x-v1	16.48% <0.00%> (+<0.01%)	⬆️
cassandra-3.x-v2	1.86% <0.00%> (+<0.01%)	⬆️
cassandra-4.x-v1	16.48% <0.00%> (+<0.01%)	⬆️
cassandra-4.x-v2	1.86% <0.00%> (+<0.01%)	⬆️
elasticsearch-7.x	1.77% <0.00%> (-0.02%)	⬇️
elasticsearch-8.x	1.77% <0.00%> (-0.02%)	⬇️
grpc_v1	9.12% <0.00%> (+<0.01%)	⬆️
grpc_v2	7.49% <0.00%> (?)
kafka	9.81% <0.00%> (+<0.01%)	⬆️
opensearch-1.x	1.77% <0.00%> (+<0.01%)	⬆️
opensearch-2.x	1.78% <0.00%> (+0.01%)	⬆️
unittests	93.99% <100.00%> (+0.01%)	⬆️

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[yurishkuro]

Thanks!