.NET 8 Support

[philliphoff]

Description

Add explicit .NET 8 targets to Dapr .NET SDK NuGet packages.

Currently, the Dapr .NET SDK NuGet packages target only .NET 6 (except for Workflow, which also targets .NET 7). With this change, the packages will additionally target .NET 8.

Issue reference

We strive to have all PR being opened based on an issue, where the problem or feature have been discussed prior to implementation.

Please reference the issue this PR will close: #1187

Checklist

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• Code compiles correctly
• Created/updated tests
• Extended the documentation

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (39a38f6) 66.42% compared to head (69e1138) 66.53%.

Files	Patch %	Lines
...AspNetCore/ActorsEndpointRouteBuilderExtensions.cs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1188      +/-   ##
==========================================
+ Coverage   66.42%   66.53%   +0.10%     
==========================================
  Files         171      171              
  Lines        5758     5752       -6     
  Branches      626      626              
==========================================
+ Hits         3825     3827       +2     
+ Misses       1784     1776       -8     
  Partials      149      149              

Flag	Coverage Δ
net6	66.52% <0.00%> (+0.09%)	⬆️
net7	66.52% <0.00%> (+0.09%)	⬆️
net8	66.52% <50.00%> (?)

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[philliphoff]

I assume codecov/patch is failing to due to the addition of the new .NET 8 target (and its codecoverage results), but I'm not familiar enough with the service to know what needs to change, if anything, to make it pass.