Adds support for Elasticsearch 7.9.1

[bowenlan-amzn]

Issue #, if available:

Description of changes:

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 #251 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #251      +/-   ##
============================================
- Coverage     78.82%   78.79%   -0.04%     
  Complexity      164      164              
============================================
  Files            91       91              
  Lines          3249     3249              
  Branches        458      458              
============================================
- Hits           2561     2560       -1     
- Misses          446      447       +1     
  Partials        242      242              

Impacted Files	Coverage Δ	Complexity Δ
.../alerting/transport/TransportIndexMonitorAction.kt	65.21% <0.00%> (-0.87%)	0.00% <0.00%> (ø%)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d54c2fb...a18d421. Read the comment docs.