Add net462 targeting

[lahma]

It's not that much of a maintenance burden and will remove some compatibility issues for folks still on full framework. SharpZipLib 2.0 could drop support if deemed so.

Also had to update testing dependencies to get NET 8 support.

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fixes #853
fixes #863

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.62%. Comparing base (c51ef18) to head (2255a47).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #872      +/-   ##
==========================================
- Coverage   74.78%   74.62%   -0.16%     
==========================================
  Files          72       75       +3     
  Lines        8535     8572      +37     
==========================================
+ Hits         6383     6397      +14     
- Misses       2152     2175      +23     

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[lahma]

@piksel can you please take a look, this PR should fix GH Actions pipelines and adds the requested net462 target which should be quite safe addition.