[Instrumentation.Wcf] Add .NET6 as target framework

[Kielek]

Fixes scenario when you referencing Instrumentation.Wcf package from .NET6+ with enabled NuGet Audit like for .NET9.
Without changes it was leading to the issues:

Workaround:
Manual referencing safe version of System.Drawing.Common, but it is not user friendly.

Changes

• Added .NET6 target (temporary, just to make a release and be able to use new version in .NET AutoInstrumnetation, just after I will bring here .NET8).
• Nullability fixes after add .net6 as a target framework

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

Attention: Patch coverage is 83.33333% with 5 lines in your changes missing coverage. Please review.

Project coverage is 78.34%. Comparing base (71655ce) to head (3643194).
Report is 554 commits behind head on main.

Files with missing lines	Patch %	Lines
...on.Wcf/Implementation/InstrumentedDuplexChannel.cs	66.66%	2 Missing ⚠️
...mplementation/HttpRequestMessagePropertyWrapper.cs	90.90%	1 Missing ⚠️
...n.Wcf/Implementation/InstrumentedRequestChannel.cs	80.00%	1 Missing ⚠️
...Wcf/Implementation/RequestTelemetryStateTracker.cs	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2243      +/-   ##
==========================================
+ Coverage   73.91%   78.34%   +4.43%     
==========================================
  Files         267       29     -238     
  Lines        9615      702    -8913     
==========================================
- Hits         7107      550    -6557     
+ Misses       2508      152    -2356     

Flag	Coverage Δ
unittests-Instrumentation.Wcf	78.34% <83.33%> (?)

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

Files with missing lines	Coverage Δ
...cf/Implementation/AsyncResultWithTelemetryState.cs	85.71% <100.00%> (ø)
...Wcf/Implementation/ClientChannelInstrumentation.cs	94.66% <100.00%> (-0.14%)	⬇️
...n.Wcf/Implementation/TelemetryPropagationWriter.cs	66.66% <100.00%> (ø)
...Implementation/WcfInstrumentationActivitySource.cs	100.00% <100.00%> (ø)
...mplementation/HttpRequestMessagePropertyWrapper.cs	70.27% <90.90%> (+7.77%)	⬆️
...n.Wcf/Implementation/InstrumentedRequestChannel.cs	68.75% <80.00%> (-6.25%)	⬇️
...Wcf/Implementation/RequestTelemetryStateTracker.cs	95.58% <75.00%> (-1.43%)	⬇️
...on.Wcf/Implementation/InstrumentedDuplexChannel.cs	59.75% <66.66%> (ø)

... and 261 files with indirect coverage changes

[CodeBlanch]

This is the part I'm not sure about. I'm going to try and find an expert on the .NET runtime team to ask about what to do here. I'll post back.

[Kielek]

Looking for any advice you can give : )

[CodeBlanch]

Haven't nailed it down yet but I approved the PR. We're adding a throw here for null EC but ExecutionContext.Run should be throwing today in that case anyway and no user has complained about it AFAIK.

We can circle back to it if any new info comes to light.

[CodeBlanch]

LGTM