[Extensions] 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 71.39%. Comparing base (71655ce) to head (b3e6192).
Report is 469 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2124      +/-   ##
==========================================
- Coverage   73.91%   71.39%   -2.53%     
==========================================
  Files         267      341      +74     
  Lines        9615    13337    +3722     
==========================================
+ Hits         7107     9522    +2415     
- Misses       2508     3815    +1307     

Flag	Coverage Δ
unittests-Exporter.Geneva	59.09% <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% <100.00%> (?)
unittests-Extensions.AWS	83.41% <ø> (?)
unittests-Extensions.Enrichment	100.00% <ø> (?)
unittests-Instrumentation.AWS	84.78% <ø> (?)
unittests-Instrumentation.AWSLambda	88.92% <ø> (?)
unittests-Instrumentation.AspNet	77.00% <ø> (?)
unittests-Instrumentation.AspNetCore	85.27% <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	82.06% <ø> (?)
unittests-Instrumentation.Owin	85.97% <ø> (?)
unittests-Instrumentation.Process	100.00% <ø> (?)
unittests-Instrumentation.Quartz	78.94% <ø> (?)
unittests-Instrumentation.Runtime	100.00% <ø> (?)
unittests-Instrumentation.SqlClient	90.90% <ø> (?)
unittests-Instrumentation.StackExchangeRedis	69.92% <ø> (?)
unittests-Instrumentation.Wcf	48.91% <ø> (?)
unittests-PersistentStorage	65.78% <ø> (?)
unittests-Resources.AWS	77.93% <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	87.61% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...try.Extensions/Trace/AutoFlushActivityProcessor.cs	82.14% <100.00%> (+4.36%)	⬆️

... and 366 files with indirect coverage changes

[CodeBlanch]

LGTM