Add missing ConfigureAwaits(false) for async using

[SimonCropp]

Found by updating configawait checker

Why this is required

[codecov-commenter]

Codecov Report

Merging #1276 (65899cd) into main (7b2e60f) will increase coverage by 1.49%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1276      +/-   ##
==========================================
+ Coverage   80.12%   81.62%   +1.49%     
==========================================
  Files         212      212              
  Lines        7029     7047      +18     
  Branches     1465     1466       +1     
==========================================
+ Hits         5632     5752     +120     
+ Misses        955      847     -108     
- Partials      442      448       +6     

Impacted Files	Coverage Δ
src/Sentry/Envelopes/Envelope.cs	97.67% <100.00%> (+1.28%)	⬆️
src/Sentry/Envelopes/EnvelopeItem.cs	91.53% <100.00%> (+1.06%)	⬆️
src/Sentry/Envelopes/JsonSerializable.cs	100.00% <100.00%> (+16.66%)	⬆️
...Sentry/Internal/Extensions/HttpClientExtensions.cs	100.00% <100.00%> (ø)
src/Sentry/Internal/Http/CachingTransport.cs	84.82% <100.00%> (+0.54%)	⬆️
...ry/Internal/Http/GzipBufferedRequestBodyHandler.cs	92.00% <100.00%> (+0.69%)	⬆️
src/Sentry/Internal/Http/HttpTransport.cs	94.89% <100.00%> (+0.03%)	⬆️
src/Sentry/Internal/SerializableExtensions.cs	100.00% <100.00%> (ø)
... and 10 more

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[bruno-garcia]

Good find! thank you