Test against OpenDistro 1.3.0 (Elasticsearch 7.3.2)

[dlvenable]

Description

• Added OpenDistro 1.3.0 (Elasticsearch 7.3.2) to the list of OpenDistro versions to test against.
• Updated the OpenSearch sink README to document that Elasticsearch 7.3.2 is the minimum working version.

Issues Resolved

N/A

Check List

• New functionality includes testing.
• New functionality has been documented.
  • New functionality has javadoc added
• Commits are signed with a real name per the DCO

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[codecov-commenter]

Codecov Report

Merging #1494 (319b4cb) into main (5c127c3) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               main    #1494   +/-   ##
=========================================
  Coverage     94.17%   94.17%           
  Complexity     1179     1179           
=========================================
  Files           165      165           
  Lines          3382     3382           
  Branches        276      276           
=========================================
  Hits           3185     3185           
  Misses          141      141           
  Partials         56       56           

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