fix(tar): enable async overrides on net6

[piksel]

Implicitly include async overrides on NETCOREAPP3_1_OR_GREATER to include them when targeting .NET 6.

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[codecov[bot]]

Codecov Report

Merging #811 (b174bc3) into master (cbb9e83) will decrease coverage by 0.04%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #811      +/-   ##
==========================================
- Coverage   74.77%   74.73%   -0.05%     
==========================================
  Files          72       72              
  Lines        8501     8507       +6     
==========================================
+ Hits         6357     6358       +1     
- Misses       2144     2149       +5     

Impacted Files	Coverage Δ
src/ICSharpCode.SharpZipLib/Tar/TarBuffer.cs	72.72% <ø> (-0.90%)	:arrow_down:
src/ICSharpCode.SharpZipLib/Tar/TarHeader.cs	84.53% <ø> (+0.05%)	:arrow_up:
src/ICSharpCode.SharpZipLib/Tar/TarInputStream.cs	62.32% <ø> (-0.89%)	:arrow_down:

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