Target .NET 8.0 and drop .NET 7.0

[filipw]

Hello 👋 This PR updates the targeting of the library.

Changes

• .NET 8.0 is now explicitly targeted by the "main" library. It was already used in tests, but the main one only targeted 6, 7 and .NET Standard 2.0.
• .NET 7.0 was removed as it reached the end of life in May 2024

Without this, using the library on .NET 8.0 means going over .NET Standard 2.0 which in turn pulls a ton of very old (dating back to 2016) dependencies via System.IO.Compression, some of which even have CVEs on them.

[twcclegg]

Thanks!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.27%. Comparing base (9ca9181) to head (494485f).
Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #264   +/-   ##
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  Coverage   75.27%   75.27%           
=======================================
  Files          34       34           
  Lines        4424     4424           
  Branches     1001     1001           
=======================================
  Hits         3330     3330           
  Misses        885      885           
  Partials      209      209           

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