fix: Added built-in System.Text.Json for netstandard2.1 target to avoid compatibility exceptions

[ElderJames]

🤔 This is a ...

• New feature
• Bug fix
• Site / documentation update
• Demo update
• Component style update
• Bundle size optimization
• Performance optimization
• Refactoring
• Code style optimization
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💡 Background and solution

📝 Changelog

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[github-actions]

[codecov]

Codecov Report

Base: 43.51% // Head: 44.98% // Increases project coverage by +1.46% 🎉

Coverage data is based on head (ebcf4f9) compared to base (e9382b8).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2922      +/-   ##
==========================================
+ Coverage   43.51%   44.98%   +1.46%     
==========================================
  Files         551      551              
  Lines       25922    25918       -4     
  Branches      263      263              
==========================================
+ Hits        11281    11659     +378     
+ Misses      14601    14219     -382     
  Partials       40       40              

Impacted Files	Coverage Δ
components/select/Select.razor.cs	53.24% <0.00%> (+0.26%)	⬆️
components/select/SelectBase.cs	71.19% <0.00%> (+0.27%)	⬆️
components/input/Input.cs	69.23% <0.00%> (+0.34%)	⬆️
components/date-picker/internal/DatePickerBase.cs	79.01% <0.00%> (+0.66%)	⬆️
components/core/CssSizeLength.cs	86.07% <0.00%> (+1.07%)	⬆️
components/grid/Col.razor.cs	96.00% <0.00%> (+1.33%)	⬆️
components/core/Extensions/JSRuntimeExtensions.cs	53.33% <0.00%> (+3.33%)	⬆️
components/grid/Row.razor.cs	84.44% <0.00%> (+8.88%)	⬆️
components/form/FormItem.razor	80.00% <0.00%> (+20.00%)	⬆️
components/core/Base/AntInputComponentBase.cs	80.95% <0.00%> (+21.90%)	⬆️
... and 9 more

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