[Shared.Tests] Replace .NET 6 target with .NET 8

[joegoldman2]

Changes

Replace .NET 6 target, which will be very soon out of support, with .NET 8.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.83%. Comparing base (71655ce) to head (55554d4).
Report is 510 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2182      +/-   ##
==========================================
- Coverage   73.91%   71.83%   -2.08%     
==========================================
  Files         267      345      +78     
  Lines        9615    13464    +3849     
==========================================
+ Hits         7107     9672    +2565     
- Misses       2508     3792    +1284     

Flag	Coverage Δ
unittests-Exporter.Geneva	59.37% <ø> (?)
unittests-Exporter.InfluxDB	95.88% <ø> (?)
unittests-Exporter.Instana	71.24% <ø> (?)
unittests-Exporter.OneCollector	94.32% <ø> (?)
unittests-Exporter.Stackdriver	75.73% <ø> (?)
unittests-Extensions	88.63% <ø> (?)
unittests-Extensions.AWS	83.41% <ø> (?)
unittests-Extensions.Enrichment	100.00% <ø> (?)
unittests-Instrumentation.AWS	84.78% <ø> (?)
unittests-Instrumentation.AWSLambda	88.96% <ø> (?)
unittests-Instrumentation.AspNet	76.73% <ø> (?)
unittests-Instrumentation.AspNetCore	70.06% <ø> (?)
unittests-Instrumentation.ConfluentKafka	14.12% <ø> (?)
unittests-Instrumentation.ElasticsearchClient	79.87% <ø> (?)
unittests-Instrumentation.EntityFrameworkCore	55.49% <ø> (?)
unittests-Instrumentation.EventCounters	76.36% <ø> (?)
unittests-Instrumentation.GrpcNetClient	79.61% <ø> (?)
unittests-Instrumentation.Hangfire	93.58% <ø> (?)
unittests-Instrumentation.Http	73.57% <ø> (?)
unittests-Instrumentation.Owin	85.97% <ø> (?)
unittests-Instrumentation.Process	100.00% <ø> (?)
unittests-Instrumentation.Quartz	78.94% <ø> (?)
unittests-Instrumentation.Runtime	97.53% <ø> (?)
unittests-Instrumentation.SqlClient	90.55% <ø> (?)
unittests-Instrumentation.StackExchangeRedis	69.92% <ø> (?)
unittests-Instrumentation.Wcf	78.47% <ø> (?)
unittests-PersistentStorage	65.78% <ø> (?)
unittests-Resources.AWS	77.85% <ø> (?)
unittests-Resources.Azure	83.89% <ø> (?)
unittests-Resources.Container	72.41% <ø> (?)
unittests-Resources.Gcp	72.54% <ø> (?)
unittests-Resources.Host	73.94% <ø> (?)
unittests-Resources.OperatingSystem	77.20% <ø> (?)
unittests-Resources.Process	100.00% <ø> (?)
unittests-Resources.ProcessRuntime	77.08% <ø> (?)
unittests-Sampler.AWS	87.74% <ø> (?)

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see 368 files with indirect coverage changes