[10.0] Migration guidance updates

[guardrex]

Fixes #36316

I think I only need one review to get this in. It's mostly just for Blazor migration sections.

The migration article uses an INCLUDES file for its Blazor content. That's the only UI stack that currently has an INCLUDES file, and we can add more if they're needed. It might be overkill, but it will ensure that future migration content on concurrent PRs won't step on each other 💥.

Side NIT 😈 that I'm fixing here: The file naming for prior migration articles can use a touch of work for consistency.

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Internal previews

📄 File	🔗 Preview link
aspnetcore/migration/20-to-21.md	aspnetcore/migration/20-to-21
aspnetcore/migration/60-to-70.md	aspnetcore/migration/60-to-70
aspnetcore/migration/70-to-80.md	aspnetcore/migration/70-to-80
aspnetcore/migration/80-to-90.md	aspnetcore/migration/80-to-90
aspnetcore/migration/90-to-100.md	aspnetcore/migration/90-to-100
aspnetcore/release-notes/aspnetcore-2.1.md	aspnetcore/release-notes/aspnetcore-2.1
aspnetcore/security/authentication/scaffold-identity.md	aspnetcore/security/authentication/scaffold-identity
aspnetcore/toc.yml	aspnetcore/toc

[Copilot]

Pull Request Overview

This PR standardizes migration documentation file naming conventions and adds comprehensive migration content for .NET 6, 7, 8, and 9 to 10 transitions. The changes support better organization and discoverability of version migration guides.

Key Changes:

• Standardizes file naming from underscores to hyphens (e.g., 20_21.md → 20-to-21.md)
• Adds new migration guides for .NET 6→7, 7→8, and 8→9 transitions
• Updates Blazor migration content for 9→10 with new feature coverage
• Establishes proper redirections for renamed files

Reviewed Changes

Copilot reviewed 7 out of 10 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
aspnetcore/toc.yml	Adds 9-to-10 migration entry and updates uid references to use hyphenated naming convention
aspnetcore/security/authentication/scaffold-identity.md	Updates xref to renamed migration file (20_21 → 20-to-21)
aspnetcore/release-notes/aspnetcore-2.1.md	Updates xref to renamed migration file (20_21 → 20-to-21)
aspnetcore/migration/90-to-100/includes/blazor.md	Expands Blazor migration content with environment configuration, boot configuration changes, and state persistence updates
aspnetcore/migration/90-to-100.md	Updates ms.date metadata to reflect latest changes
aspnetcore/migration/80-to-90.md	New comprehensive migration guide covering .NET 8 to 9 transition including static assets, Blazor updates, and breaking changes
aspnetcore/migration/70-to-80.md	New comprehensive migration guide covering .NET 7 to 8 transition including Blazor Web App conversions and Docker updates
aspnetcore/migration/60-to-70.md	New comprehensive migration guide covering .NET 6 to 7 transition including simplified component binding and authentication changes
aspnetcore/migration/20-to-21.md	Updates uid from underscored to hyphenated format for consistency
.openpublishing.redirection.json	Adds redirections for all renamed and relocated migration files to prevent broken links

[wadepickett]

Looks great! I can't find any issues. Approved.