[WIP] Support .NET 10

[martincostello]

Changes

Add support for building and testing with .NET 10.

Initially using .NET 10 preview 4, will update as previews progress.

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 86.74%. Comparing base (3f86f47) to head (d7b4e05).

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6307      +/-   ##
==========================================
- Coverage   86.76%   86.74%   -0.02%     
==========================================
  Files         258      258              
  Lines       11879    11879              
==========================================
- Hits        10307    10305       -2     
- Misses       1572     1574       +2     

Flag	Coverage Δ
unittests-Project-Experimental	86.65% <ø> (+0.33%)	⬆️
unittests-Project-Stable	86.34% <ø> (-0.32%)	⬇️
unittests-Solution	86.72% <ø> (+0.28%)	⬆️
unittests-UnstableCoreLibraries-Experimental	85.78% <ø> (ø)

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see 4 files with indirect coverage changes

[martincostello]

I don't know why the w3c-trace-context integration tests are failing.

[martincostello]

I don't know why the w3c-trace-context integration tests are failing.

Caused by this breaking change - just need to work out it seems to be applying to .NET 8, 9 & 10 in the W3C tests and why manually setting it back isn't working.

[martincostello]

just need to work out it seems to be applying to .NET 8, 9 & 10 in the W3C tests

Fixed the .NET 10 tests - the .NET 8 and 9 tests are failing because the .NET 8 and 9 runtimes aren't present in the docker image, so it rolls forward to .NET 10, then the breaking change applies (but can't be turned off because the new API is missing), so the tests fail.

[martincostello]

Fixed the .NET 10 tests - the .NET 8 and 9 tests are failing because the .NET 8 and 9 runtimes aren't present in the docker image, so it rolls forward to .NET 10, then the breaking change applies (but can't be turned off because the new API is missing), so the tests fail.

This wasn't correct - the actual reason was that all TFMs were referencing the 10.0.* package, so the breaking change was being applied. The fix was to use the workaround with all the TFMs.

Might need tweaking depending on the outcome of #6327.