Adds support for Elasticsearch 7.9

[dbbaughe]

Issue #, if available:

Description of changes:
Adds support for Elasticsearch 7.9

Note: This will break until Job Scheduler SPI and Notification are published to Maven

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

Merging #283 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #283      +/-   ##
============================================
- Coverage     75.29%   75.26%   -0.03%     
- Complexity      703      704       +1     
============================================
  Files            89       89              
  Lines          3914     3914              
  Branches        627      627              
============================================
- Hits           2947     2946       -1     
  Misses          641      641              
- Partials        326      327       +1     

Impacted Files	Coverage Δ	Complexity Δ
...ticsearch/indexstatemanagement/model/Transition.kt	80.39% <0.00%> (-5.89%)	5.00% <0.00%> (ø%)
...ndexstatemanagement/IndexStateManagementHistory.kt	77.66% <0.00%> (+1.94%)	28.00% <0.00%> (+1.00%)

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[dbbaughe]

Notification and JS SPI were published to Maven, tests pass now.