fix(migrations): Split v21.0.0 migration into two separate migrations

[Copilot]

Closes #

Schemas are not supported on Angular CLI migrations. The previous implementation used a schema with x-prompt to ask users about import migration, which doesn't work as intended.

Changes

Split the single v21.0.0 migration into two separate migrations:

• migration-50: Standard migration (theme updates, etc.) - runs automatically
• migration-51: Optional import migration to granular entry points - marked with optional: true

Removed schema.json from update-21_0_0 directory. Extracted all import migration logic into new update-21_0_0_import-migration directory.

Users can now run the optional import migration explicitly:

ng update igniteui-angular --migrate-only --from=20.1.0 --to=21.0.0 --name=migration-51

Additional information (check all that apply):

• Bug fix
• New functionality
• Documentation
• Demos
• CI/CD
• Refactoring

Checklist:

• All relevant tags have been applied to this PR
• This PR includes unit tests covering all the new code (test guidelines)
• This PR includes API docs for newly added methods/properties (api docs guidelines)
• This PR includes feature/README.MD updates for the feature docs
• This PR includes general feature table updates in the root README.MD
• This PR includes CHANGELOG.MD updates for newly added functionality
• This PR contains breaking changes
• This PR includes ng update migrations for the breaking changes (migrations guidelines)
• This PR includes behavioral changes and the feature specification has been updated with them

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