Modernize the build

[berezovskyi]

• update dotnetrdf version only to the latest 1.x for now
• upgrade the GH Action, see https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/ and https://github.com/microsoft/setup-msbuild
• update log4net version
• Update https://www.nuget.org/downloads executable
• set RestorePackages to true and RequireRestoreConsent in the NuGet.targets
• deprecate OSLC/JSON provider
• Switch tests to MSTestV2 #66
• Publish core libs and the client on Nuget #65
• Port our MediaTypeFormatter to be a TextOutputFormatter #67
• target both .NET Framework 4.5 (at least, the client) and .NET Standard 2, similar to dotNetRDF #69
• Consider generating domain classes like in Lyo with LyoD #68

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New 2023 items:

• update dotNetRdf at least to 1.0.0.2533 (a mix of that version and 0.9.0.2110 was encountered, downgrade was made temporarily to keep things uniform) updated to 1.0.13

[berezovskyi]

And also see https://news.ycombinator.com/item?id=30331674

[berezovskyi]

May be of interest: dotnetrdf/dotnetrdf#537

[berezovskyi]

I consider the task completed as the build now work with the most recent nuget/msbuild GH Actions from MSFT. .NET Standard/Core migration is a separate issue as well as other things, which were extracted to individual issues.