.NET 10 preparation

[martincostello]

Cherry-pick refactoring from #3283 to make multi-targeting easier for ASP.NET Core 10 and Microsoft.OpenApi v2.

This PR will be easiest to review one commit at a time, as the second commit effectively just renames all the non-TFM'd snapshots to one of the TFMs, then the other files are just copies of the those with the name of the other TFM.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 70.45455% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.56%. Comparing base (1d7aed0) to head (76f789d).
⚠️ Report is 244 commits behind head on master.

Files with missing lines	Patch %	Lines
...re.SwaggerGen/SwaggerGenerator/SwaggerGenerator.cs	68.75%	5 Missing ⚠️
...ggerGen/SchemaGenerator/OpenApiSchemaExtensions.cs	55.55%	4 Missing ⚠️
...Core.SwaggerGen/SchemaGenerator/SchemaGenerator.cs	83.33%	2 Missing ⚠️
...pNetCore.Annotations/AnnotationsOperationFilter.cs	0.00%	1 Missing ⚠️
...e.SwaggerGen/SwaggerGenerator/OpenApiAnyFactory.cs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3285      +/-   ##
==========================================
- Coverage   86.08%   83.56%   -2.53%     
==========================================
  Files          79       77       -2     
  Lines        3378     3224     -154     
  Branches      588      559      -29     
==========================================
- Hits         2908     2694     -214     
- Misses        470      530      +60     

Flag	Coverage Δ
Linux	83.56% <70.45%> (ø)
Windows	83.56% <70.45%> (+45.32%)	⬆️
macOS	83.56% <70.45%> (ø)

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