remove .NET5 (out of support runtime) from test projects

[heavenwing]

Description

remove .NET5 TargetFrameworks from test projects, because .NET5 is out of support runtime.

Issue reference

N/A

Checklist

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

• [x] Code compiles correctly
• [ ] Created/updated tests
• [ ] Extended the documentation

[codecov[bot]]

Codecov Report

Merging #955 (5582436) into master (37ed498) will decrease coverage by 0.01%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #955      +/-   ##
==========================================
- Coverage   69.49%   69.47%   -0.02%     
==========================================
  Files         155      155              
  Lines        5130     5130              
  Branches      554      554              
==========================================
- Hits         3565     3564       -1     
- Misses       1434     1435       +1     
  Partials      131      131              

Flag	Coverage Δ
net5	?
net6	69.39% <ø> (ø)
netcoreapp3.1	69.43% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/Dapr.Client/DaprClientGrpc.cs	86.57% <0.00%> (-0.16%)	:arrow_down:

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[halspang]

@heavenwing - Thanks for this PR! However, we'd want to give more warning about removing a build target. These are also only test targets, so we're making sure we still support that version.

Instead of just the PR, can you create an issue and we can prioritize it through the standard milestone process?

[halspang]

I actually ended up doing this in https://github.com/dapr/dotnet-sdk/pull/1005.