chore!(deps): use .NET 8 libraries on netstandard2.0

[nogic1008]

This is a BREAKING CHANGE because System.Text.Json>=7.0.0 warns on .NET Core 3.1 or lower. (see dotnet/runtime#94327)

To suppress warning, you can add SuppressTfmSupportBuildWarnings on your csproj file.

Warning

This library does not been tested on .NET Core 3.1 or lower environments.
Be sure to integration test yourself.

<SuppressTfmSupportBuildWarnings>true</SuppressTfmSupportBuildWarnings>

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.94%. Comparing base (bda607d) to head (3e00b46).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #288   +/-   ##
=======================================
  Coverage   98.94%   98.94%           
=======================================
  Files           2        2           
  Lines          95       95           
  Branches        8        8           
=======================================
  Hits           94       94           
  Partials        1        1           

Flag	Coverage Δ
Linux	98.83% <ø> (ø)
Windows	98.94% <ø> (ø)
macOS	98.83% <ø> (ø)
net48	98.88% <ø> (ø)
net6.0	98.83% <ø> (ø)
net8.0	98.83% <ø> (ø)

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