Run CI also on oldest compatible Julia version and update GH actions

[devmotion]

I noticed that currently CI only tests the latest stable Julia release and nightly. To avoid accidentally breaking support for older, officially declared to be compatible, Julia versions I think it would be good to run CI test also with the oldest compatible Julia version (currently 1.6).

I used the opportunity and also updated and simplified the GH actions.

[pkofod]

Thanks. Let's see if it works :)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.07%. Comparing base (7cc8328) to head (3a31db9).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1090      +/-   ##
==========================================
- Coverage   85.43%   85.07%   -0.37%     
==========================================
  Files          45       46       +1     
  Lines        3276     3498     +222     
==========================================
+ Hits         2799     2976     +177     
- Misses        477      522      +45     

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