update links that result in 404 results

[timdeschryver]

Hello, I hope it's OK to create this PR.
If not, feel free to just close it without hard feelings.

I've built a tool at work that traverses a repository to find broken links, and I also wanted to try it out here 😅 Hence this PR...

There are more links broken, but these are mainly in readme files of old samples (ASP.NET v3-5).

The big portion of this PR is:

• Updating links to readme files => these should contain *.md at the end
• https://owin.org => http://owin.org because the certificate has become invalid
• Update references to deleted samples.

The links I couldn't find a new version for:

• https://github.com/aspnet/AspLabs/tree/main/src/BlazorWebAssemblyCustomInitialization which results in a 404 because the sample got deleted in Removed old Blazor-related experiments aspnet/AspLabs#616.
• The links referring to https://osi-model.com

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Internal previews

📄 File	🔗 Preview link
aspnetcore/fundamentals/app-state.md	aspnetcore/fundamentals/app-state
aspnetcore/fundamentals/owin.md	aspnetcore/fundamentals/owin
aspnetcore/fundamentals/servers/index.md	aspnetcore/fundamentals/servers/index
aspnetcore/fundamentals/servers/yarp/distributed-tracing.md	aspnetcore/fundamentals/servers/yarp/distributed-tracing
aspnetcore/migration/proper-to-2x/mvc2.md	aspnetcore/migration/proper-to-2x/mvc2
aspnetcore/test/dev-tunnels.md	aspnetcore/test/dev-tunnels
CONTRIBUTING.md	CONTRIBUTING

[guardrex]

Thanks @timdeschryver! 🎷

[guardrex]

I'll take care of the Blazor link by EOD. I opened #35294 to fix it.

[guardrex]

We don't like to link to Widipedia ...

https://en.wikipedia.org/wiki/OSI_model

However, another MS doc set does ...

https://learn.microsoft.com/en-us/windows-hardware/drivers/network/windows-network-architecture-and-the-osi-model

[guardrex]

I'll take care of the OSI links. I opened an issue for that, too, at #35295.