Use Microsoft.Bcl.AsyncInterfaces for net461 and netstandard2.0

[clairernovotny]

Fixes #897

[codecov]

Codecov Report

Merging #900 into master will decrease coverage by 2.63%.
The diff coverage is 47.36%.

@@            Coverage Diff             @@
##           master     #900      +/-   ##
==========================================
- Coverage   65.51%   62.87%   -2.64%     
==========================================
  Files         168      163       -5     
  Lines       30166    28710    -1456     
  Branches    20523    19800     -723     
==========================================
- Hits        19762    18052    -1710     
+ Misses       2499     2119     -380     
- Partials     7905     8539     +634

Flag	Coverage Δ
#ixnet	62.87% <47.36%> (-2.64%)	⬇️

Impacted Files	Coverage Δ
...tive.Async/System/Linq/Operators/IgnoreElements.cs	50% <0%> (-11.85%)	⬇️
.../System.Linq.Async/System/Linq/Operators/Lookup.cs	83.33% <0%> (ø)	⬆️
...e/System.Linq.Async/System/Linq/Operators/Union.cs	59.17% <0%> (-10.87%)	⬇️
....Interactive.Async/System/Linq/Operators/Concat.cs	41.22% <0%> (-8.17%)	⬇️
....Async/System/Linq/Operators/ElementAtOrDefault.cs	77.08% <0%> (ø)	⬆️
....Interactive.Async/System/Linq/Operators/Repeat.cs	44.44% <0%> (-11.39%)	⬇️
...m.Linq.Async/System/Linq/AsyncEnumerableHelpers.cs	62.16% <0%> (ø)
...stem.Linq.Async/System/Linq/Operators/Aggregate.cs	60% <0%> (ø)	⬆️
...m.Interactive.Async/System/Linq/Operators/Catch.cs	37.41% <0%> (-12.72%)	⬇️
...stem.Linq.Async/System/Linq/Operators/Utilities.cs	50% <0%> (ø)
... and 92 more

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