remove minimum C++ version, update minimum R version to 4.0

[gmweaver]

Updating for release, dropping support for version prior to 4.0.

Also, updated CI to use GH actions instead of Travis CI and Appveyor.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.41%. Comparing base (5064978) to head (9fab14f).
Report is 4 commits behind head on master.

❗ Current head 9fab14f differs from pull request most recent head 79428f7

Please upload reports for the commit 79428f7 to get more accurate results.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #42      +/-   ##
==========================================
+ Coverage   87.35%   87.41%   +0.06%     
==========================================
  Files          18       18              
  Lines        1700     1701       +1     
==========================================
+ Hits         1485     1487       +2     
+ Misses        215      214       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.