Added Elasticsearch 6.8 support to Document Migrations

[chelma]

Description

• Updated the Doc Migrations code to accept a source version CLI argument
• Centralized some behavior with how we construct parser classes specific to source versions
• Updated unit tests where appropriate
• Added some additional test snapshots. One (ES_6_8_Updates_Deletes_Native) is currently used in our unit tests, while the other two (ES_5_6_Updates_Deletes, ES_6_8_Updates_Deletes_Merged) are not currently used, but will be used when we support Lucene Version hot-swapping (https://opensearch.atlassian.net/browse/MIGRATIONS-1965) and the ability to handle Indices originally created on an Elastic 5.X cluster in an Elastic 6.X source snapshot.
• Added the ability to spin up an Elastic 5.6 source cluster via docker compose

Issues Resolved

• https://opensearch.atlassian.net/browse/MIGRATIONS-1953

Testing

Lots of manual testing with Docker Compose; unit testing

Check List

• New functionality includes testing
  • All tests pass, including unit test, integration test and doctest
• New functionality has been documented
• Commits are signed per the DCO using --signoff

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[codecov]

Codecov Report

Attention: Patch coverage is 48.48485% with 17 lines in your changes missing coverage. Please review.

Project coverage is 72.08%. Comparing base (26a74b4) to head (f254c9f).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...S/src/main/java/com/rfs/common/ClusterVersion.java	44.44%	5 Missing ⚠️
.../com/rfs/common/SourceResourceProviderFactory.java	33.33%	3 Missing and 1 partial ⚠️
.../version_es_6_8/SourceResourceProvider_ES_6_8.java	55.55%	4 Missing ⚠️
...ersion_es_7_10/SourceResourceProvider_ES_7_10.java	55.55%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #921      +/-   ##
============================================
- Coverage     74.51%   72.08%   -2.43%     
+ Complexity     1961     1869      -92     
============================================
  Files           343      346       +3     
  Lines         13555    13586      +31     
  Branches        786      786              
============================================
- Hits          10101     9794     -307     
- Misses         2964     3338     +374     
+ Partials        490      454      -36     

Flag	Coverage Δ
gradle-test	65.64% <48.48%> (-3.36%)	⬇️
python-test	88.58% <ø> (ø)

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