[WIP] v2: Migrate ECS back-end to Harmony, drop Python <=3.9, & other breaking changes

[mfisher87]

Closes #530

ECS is being decommissioned. To enable icepyx to continue working, we'll release a v2 which uses Harmony as the back-end. v1 will cease to function. We will add a deprecation warning to v1 on an upcoming release.

Breaking changes

• Remove support for variable subsetting
• ?

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.37%. Comparing base (a762ad8) to head (817175b).
Report is 30 commits behind head on development.

Additional details and impacted files

@@             Coverage Diff              @@
##           development     #544   +/-   ##
============================================
  Coverage        65.37%   65.37%           
============================================
  Files               36       36           
  Lines             3038     3038           
  Branches           531      531           
============================================
  Hits              1986     1986           
  Misses             970      970           
  Partials            82       82           

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[JessicaS11]

Should we close this in favor of #613?

[mfisher87]

Yes, good call. This is a vestige of when we planned on working on a v2 branch.