zip: finish stream async when async

[piksel]

This PR fixes the async finishing of entries in ZipOutputStream. The initial problem was that there was a sneaky base.Finish() call in WriteEntryFooter which in turn wrote synchronously to the output stream when called. Additionally, DeflateOutputSteam.AsyncDispose was not included when targeting .NET 6 due to the preprocessor constants for NETSTANDARD2_1_OR_GREATER not being defined.

This also adds tests that ensures that writing zip streams can be done wholly async, without calling any of the output streams synchronous methods.

Fixes #801

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[codecov[bot]]

Codecov Report

Merging #808 (759570e) into master (cdd0931) will increase coverage by 0.05%. The diff coverage is 93.33%.

@@            Coverage Diff             @@
##           master     #808      +/-   ##
==========================================
+ Coverage   74.72%   74.77%   +0.05%     
==========================================
  Files          72       72              
  Lines        8485     8501      +16     
==========================================
+ Hits         6340     6357      +17     
+ Misses       2145     2144       -1     

Impacted Files	Coverage Δ
...ib/Zip/Compression/Streams/DeflaterOutputStream.cs	82.72% <ø> (+1.72%)	:arrow_up:
src/ICSharpCode.SharpZipLib/Zip/ZipOutputStream.cs	88.57% <93.33%> (+0.70%)	:arrow_up:
src/ICSharpCode.SharpZipLib/Zip/ZipInputStream.cs	82.87% <0.00%> (-0.16%)	:arrow_down:

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