Remove netstandard 1.3 target

[JimBobSquarePants]

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• I have provided test coverage for my change (where applicable)

Description

See #1886 (comment)

Almost no actual code changes on the back of this but exposes a ton of new APIs to us.

[brianpopow]

There is still a #if NETSTANDARD1_3 in Guard.cs leftover

[codecov]

Codecov Report

Merging #1888 (436b8a0) into master (ae7b995) will decrease coverage by 0%.
The diff coverage is 0%.

@@          Coverage Diff           @@
##           master   #1888   +/-   ##
======================================
- Coverage      87%     87%   -1%     
======================================
  Files         957     957           
  Lines       50393   50389    -4     
  Branches     6263    6263           
======================================
- Hits        44180   44176    -4     
  Misses       5181    5181           
  Partials     1032    1032           

Flag	Coverage Δ
unittests	87% <0%> (-1%)	⬇️

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Impacted Files	Coverage Δ
src/ImageSharp/Common/Helpers/Guard.cs	25% <0%> (-38%)	⬇️
.../Tiff/Compression/Compressors/DeflateCompressor.cs	100% <ø> (ø)

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[JimBobSquarePants]

That’s loaded via the submodule no? I’ll double check.

EDIT.

Found it and removed. Good catch!