Upgrade to Microsoft.OpenApi v2

[martincostello]

Upgrade to Microsoft.OpenApi v2, which would unlock support for OpenAPI 3.1.

Contributes to #2349.

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This currently targets preview 5 and compiles, but lots of tests are broken.

The migration was guided by the release notes for the first preview.

Some might be bugs in the upgrade, others might be bugs in Microsoft.OpenApi v2 itself. I'll need to investigate further, then I can open any upstream issues as neccessary.

The most worrying failure so far is this one:

System.MissingMethodException: Method not found: 'Void Microsoft.OpenApi.Models.OpenApiOperation.set_Tags(System.Collections.Generic.IList`1<Microsoft.OpenApi.Models.OpenApiTag>)'.
   at Microsoft.AspNetCore.OpenApi.OpenApiGenerator.GetOperation(String httpMethod, MethodInfo methodInfo, EndpointMetadataCollection metadata, RoutePattern pattern)
   at Microsoft.AspNetCore.OpenApi.OpenApiGenerator.GetOpenApiOperation(MethodInfo methodInfo, EndpointMetadataCollection metadata, RoutePattern pattern)
   at Microsoft.AspNetCore.Builder.OpenApiEndpointConventionBuilderExtensions.AddAndConfigureOperationForEndpoint(EndpointBuilder endpointBuilder, Func`2 configure)
   at Microsoft.AspNetCore.Builder.OpenApiEndpointConventionBuilderExtensions.<>c__0`1.<WithOpenApi>b__0_0(EndpointBuilder builder)
   at Microsoft.AspNetCore.Routing.RouteEndpointDataSource.CreateRouteEndpointBuilder(RouteEntry entry, RoutePattern groupPrefix, IReadOnlyList`1 groupConventions, IReadOnlyList`1 groupFinallyConventions)
   at Microsoft.AspNetCore.Routing.RouteEndpointDataSource.GetGroupedEndpoints(RouteGroupContext context)
   at Microsoft.AspNetCore.Routing.RouteGroupBuilder.GroupEndpointDataSource.GetGroupedEndpointsWithNullablePrefix(RoutePattern prefix, IReadOnlyList`1 conventions, IReadOnlyList`1 finallyConventions, IServiceProvider applicationServices)
   at Microsoft.AspNetCore.Routing.RouteGroupBuilder.GroupEndpointDataSource.get_Endpoints()
   at Microsoft.AspNetCore.Routing.CompositeEndpointDataSource.CreateEndpointsUnsynchronized()
   at Microsoft.AspNetCore.Routing.CompositeEndpointDataSource.EnsureEndpointsInitialized()
   at Microsoft.AspNetCore.Routing.CompositeEndpointDataSource.get_Endpoints()
   at Microsoft.AspNetCore.Routing.DataSourceDependentCache`1.Initialize()
   at System.Threading.LazyInitializer.EnsureInitializedCore[T](T& target, Boolean& initialized, Object& syncLock, Func`1 valueFactory)
   at Microsoft.AspNetCore.Routing.Matching.DataSourceDependentMatcher..ctor(EndpointDataSource dataSource, Lifetime lifetime, Func`1 matcherBuilderFactory)
   at Microsoft.AspNetCore.Routing.Matching.DfaMatcherFactory.CreateMatcher(EndpointDataSource dataSource)
   at Microsoft.AspNetCore.Routing.EndpointRoutingMiddleware.InitializeCoreAsync()
--- End of stack trace from previous location ---
   at Microsoft.AspNetCore.Routing.EndpointRoutingMiddleware.<Invoke>g__AwaitMatcher|10_0(EndpointRoutingMiddleware middleware, HttpContext httpContext, Task`1 matcherTask)
   at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddlewareImpl.Invoke(HttpContext context)

This suggests that the changes are so breaking that it is basically impossible to use Microsoft.OpenApi v2 with ASP.NET Core versions earlier than 10. This creates a difficult situation for us, as we would potentially need to conditionally compile across TFMs and only be able to support OpenAPI 3.1 with net10.0 and need to stay on 1.x for lower TFMs while we still support them. This could get quite messy and complex quite quickly.

Maybe it's not actually that bad and it's an artifact of how our integration test suite is set up, but it's not an encouraging start. We can get further insight on that once ASP.NET Core 10 Preview 1 is available, as then this PR can be updated to additionally target net10.0 as that has support for OpenAPI 3.1.

/cc @captainsafia For thoughts on the above.

~~Also applies some refactorings in files that needed to be touched, but those will be extracted out and merged ahead of time where possible in a future separate PR.~~ #3254