Cascading values/params and render mode boundaries 8.0

[guardrex]

Fixes #30947
Addresses #28161

Thanks @gregoryagu! 🎷 ... This needed a lot of work. I'm glad you called it out.

• I'm adding a whole section based on what Steve wrote, and I'm cross-linking that in the Migration topic.
• I fix up the existing example ...
  • They can use the Routes component with full interactive server rendering ... but at the cost of adopting it across the app. This approach fine for an example that neatly replaces the layout approach of <8.0 ... AND I place a NOTE there to cover the alternative with a root-level cascading value, which includes a code example.
  • We already had a section on using root-level cascading values.

Do you want to look all of this over before I ping Steve?

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Internal previews

📄 File	🔗 Preview link
aspnetcore/blazor/components/cascading-values-and-parameters.md	ASP.NET Core Blazor cascading values and parameters
aspnetcore/blazor/components/render-modes.md	ASP.NET Core Blazor render modes
aspnetcore/migration/70-80.md	Migrate from ASP.NET Core 7.0 to 8.0

[gregoryagu]

Looks good to me!

[guardrex]

Thanks, I'll send Steve a message and see if he has time to take a look.

UPDATE: Done! ... If he's busy tho, I'm going to go ahead and merge this. I'm 🏃😰 trying to get everything done and can't be held up too much right now. The PRs start stepping on each other 😈 under high doc churn if I don't get them in quickly, and the other doc authors might need to get into the Migration article.