Drop Newtonsoft.Json and unsupported TFMs

[martincostello]

• Remove net461 and net5.0 TFMs, as those versions are no longer supported.
• Add net6.0 TFM.
• Remove dependency on Newtonsoft.Json and just use System.Text.Json.
• Update NuGet packages to their latest, or more recent, versions.
• Add async overload for registering an HTTP bundle.
• Remove redundant cast needed from net5.0 TFM.

Resolves #441.
Resolves #442.

[codecov-commenter]

Codecov Report

Merging #444 (c8a5646) into main (0cbb0b0) will decrease coverage by 0.14%.
The diff coverage is 95.45%.

❗ Current head c8a5646 differs from pull request most recent head bf6b9ea. Consider uploading reports for the commit bf6b9ea to get more accurate results

@@             Coverage Diff             @@
##              main     #444      +/-   ##
===========================================
- Coverage   100.00%   99.85%   -0.15%     
===========================================
  Files           16       15       -1     
  Lines          704      705       +1     
===========================================
  Hits           704      704              
- Misses           0        1       +1     

Flag	Coverage Δ
linux	99.85% <95.45%> (-0.15%)	⬇️
macos	99.85% <95.45%> (-0.15%)	⬇️
windows	99.85% <95.45%> (-0.15%)	⬇️

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Impacted Files	Coverage Δ
src/HttpClientInterception/Bundles/Bundle.cs	100.00% <ø> (ø)
src/HttpClientInterception/BundleExtensions.cs	96.66% <90.90%> (-3.34%)	⬇️
...rc/HttpClientInterception/Bundles/BundleFactory.cs	100.00% <100.00%> (ø)
src/HttpClientInterception/Bundles/BundleItem.cs	100.00% <100.00%> (ø)
...pClientInterception/Bundles/BundleItemConverter.cs	100.00% <100.00%> (ø)
...rception/HttpClientInterceptorOptionsExtensions.cs	100.00% <100.00%> (ø)
...eption/HttpRequestInterceptionBuilderExtensions.cs	100.00% <100.00%> (ø)

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