ASP.NET Core (and other) cross-link updates

[guardrex]

Fixes #3004

The primary goal is to remove the version pinning to ASP.NET Core docs. I also ...

• Changed the site domain (docs.microsoft.com 👉 learn.microsoft.com).
• Dropped the culture segments (i.e., en-us).

I didn't test all of the links, and I only performed a little work with text around links and link text.

[jennyf19]

This is great, thanks @guardrex

[kllysng]

A couple small nits. Checked many of the new links and they look good. Thanks!

[jmprieur]

LGTM
Thanks @guardrex
Very nice!