update ES versions we test against

[jameslamb]

Working on #161 forced me to think about the versions we test against and the guarantees we make to our users. In this PR, I want to propose a new set of versions to test against and a documented strategy for choosing them.

I also created #168 to document some room for improvement beyond this PR.

@skirmer since you've been a user of uptasticsearch for so long, I'd love your feedback on the proposal here if you have time to take a look (focus on CONTRIBUTING.md). Thanks!

[skirmer]

Looks alright to me, I think the additional support will be helpful.

[codecov-io]

Codecov Report

Merging #169 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #169   +/-   ##
======================================
  Coverage    93.1%   93.1%           
======================================
  Files           8       8           
  Lines         595     595           
======================================
  Hits          554     554           
  Misses         41      41

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