[repo] Remove .NET 7 target

[Kielek]

Follow up to open-telemetry/opentelemetry-dotnet#5795
Preparation to #2034 (it solves at least some issues from the list).

Changes

Remove .NET 7 target

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 75.43%. Comparing base (71655ce) to head (67db15e).
Report is 404 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2038      +/-   ##
==========================================
+ Coverage   73.91%   75.43%   +1.51%     
==========================================
  Files         267      314      +47     
  Lines        9615    12057    +2442     
==========================================
+ Hits         7107     9095    +1988     
- Misses       2508     2962     +454     

Flag	Coverage Δ
unittests-Exporter.Geneva	64.27% <ø> (?)
unittests-Exporter.InfluxDB	95.88% <ø> (?)
unittests-Exporter.Instana	71.24% <ø> (?)
unittests-Exporter.OneCollector	93.07% <ø> (?)
unittests-Exporter.Stackdriver	75.73% <ø> (?)
unittests-Extensions	88.57% <ø> (?)
unittests-Extensions.AWS	83.41% <ø> (?)
unittests-Extensions.Enrichment	100.00% <ø> (?)
unittests-Instrumentation.AWS	75.77% <ø> (?)
unittests-Instrumentation.AWSLambda	88.92% <ø> (?)
unittests-Instrumentation.AspNet	76.73% <ø> (?)
unittests-Instrumentation.AspNetCore	85.27% <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	85.79% <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	67.02% <ø> (?)
unittests-Instrumentation.Wcf	48.91% <ø> (?)
unittests-PersistentStorage	65.78% <ø> (?)
unittests-Resources.AWS	77.93% <ø> (?)
unittests-Resources.Azure	82.83% <ø> (?)
unittests-Resources.Container	72.41% <ø> (?)
unittests-Resources.Gcp	72.54% <ø> (?)
unittests-Resources.Host	72.64% <ø> (?)
unittests-Resources.OperatingSystem	79.68% <ø> (?)
unittests-Resources.Process	100.00% <ø> (?)
unittests-Resources.ProcessRuntime	77.08% <ø> (?)
unittests-Sampler.AWS	87.97% <ø> (?)

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

Files with missing lines	Coverage Δ
...ry.Exporter.Geneva/Metrics/GenevaMetricExporter.cs	97.05% <ø> (+30.39%)	⬆️
....Exporter.OneCollector/Internal/CallbackManager.cs	96.00% <ø> (+0.16%)	⬆️
...Internal/Serialization/BatchSerializationResult.cs	100.00% <ø> (ø)
...rialization/CommonSchemaJsonSerializationHelper.cs	100.00% <ø> (ø)
...lector/Internal/Transports/TransportSendRequest.cs	100.00% <ø> (ø)
...llector/OneCollectorExporterValidationException.cs	25.00% <ø> (ø)

... and 328 files with indirect coverage changes

[CodeBlanch]

LGTM