Invocation of dotnet-getdocument.dll tool from Microsoft.Extensions.ApiDescription.Server.targets may result in trailing backslash and broken build on Windows

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Summary of the changes (Less than 80 chars)

Description

Value of OpenApiDocumentsDirectory must be trimmed of trailing backslash on Windows after path normalization.
OpenApiDocumentsDirectory = ../foo/ -> Path.GetFullPath (on Windows) -> <rooted Path>\foo\

Move TrimEnd('\') to after path normalization, as a Linux style directory separator would not be trimmed, translated to windows style backslash by the path normalization done by GetFullPath if build runs on Windows. Resulting value in _DotNetGetDocumentOutputPath would still have trailing backslash and break the command line (on Windows).

There seem to be some changes to this in past commits, moving the TrimEnd around. Looks like a regression.

Fixes #59749 (in this specific format)