Adopt File-scoped namespaces

[martincostello]

While working on #3283 the IDE was marking lots of suggestions to modernise the codebase.

Rather than ignore it, or make such changes part of the .NET 10 adoption, instead just go through everything now and spruce it up.

This PR adopts file-scoped namespaces for all of the code files, and also simplifies the #if ... pre-processor directives.

Post-merge there will be a PR to add the squashed commit to .git-blame-ignore-revs. #3332

PR commits are grouped in chunks for easier review if anyone wants to.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 84.89755% with 457 lines in your changes missing coverage. Please review.

Project coverage is 84.82%. Comparing base (99ccafa) to head (1abac7d).

Files with missing lines	Patch %	Lines
...DependencyInjection/SwaggerGenOptionsExtensions.cs	55.95%	85 Missing ⚠️
...ggerGen/SchemaGenerator/OpenApiSchemaExtensions.cs	44.88%	70 Missing ⚠️
...re.SwaggerGen/SwaggerGenerator/SwaggerGenerator.cs	92.25%	46 Missing ⚠️
...waggerGen/XmlComments/XmlCommentsDocumentFilter.cs	21.73%	18 Missing ⚠️
...gerGen/XmlComments/XmlCommentsRequestBodyFilter.cs	75.36%	17 Missing ⚠️
...emaGenerator/JsonSerializerDataContractResolver.cs	90.69%	16 Missing ⚠️
...e.SwaggerGen/SwaggerGenerator/OpenApiAnyFactory.cs	52.94%	16 Missing ⚠️
...AspNetCore.SwaggerUI/SwaggerUIOptionsExtensions.cs	77.77%	16 Missing ⚠️
...SwaggerGen/SchemaGenerator/MemberInfoExtensions.cs	58.06%	13 Missing ⚠️
...Core.SwaggerGen/SchemaGenerator/SchemaGenerator.cs	94.92%	13 Missing ⚠️
... and 38 more

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3331      +/-   ##
==========================================
- Coverage   84.84%   84.82%   -0.03%     
==========================================
  Files          77       79       +2     
  Lines        3221     3216       -5     
  Branches      556      556              
==========================================
- Hits         2733     2728       -5     
  Misses        488      488              

Flag	Coverage Δ
Linux	83.51% <83.74%> (-0.03%)	⬇️
Windows	66.26% <67.21%> (-0.06%)	⬇️
macOS	83.51% <83.74%> (-0.03%)	⬇️

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