Streamlining the migration experience

### Command

update

### Description

I might be missing something but it seems to me that Angular provides three kinds of "migrations":

- **mandatory** migrations (e.g. `update-workspace-config`)
- **optional** migrations (e.g. `use-application-builder`)
- "**manual** migrations" implemented as schematics (e.g. `signal-input-migration`)

While everything works perfectly, like Swiss clocks, the CLI could provide a more unified experience to thwart the following limitations :
- users have to run `ng update` to list which packages could be updated before running the same command with the packages they want to update
- to run optional migrations later, users have to run `ng update --migrate-only --from [previous_version]`
- to list "manual migrations", users have to run `ng g @angular/core: --help`
- there is no built-in way to implement a "check" CI step that prevents making changes with code that should have been migrated (e.g. running migrations and checking that the output is `Nothing to be done.`)

### Describe the solution you'd like

It would be nice if `ng update` could:
- merge both optional and "manual migrations" / migration schematics concepts
- have a mode that migrates all packages that can be migrated
- run migrations without interaction or by only prompting users if the codebase contains code that should be migrated (i.e. after running [tsurge's analyze phase](https://github.com/angular/angular/blob/3d86c586dc73a426f7bda45472a6e2c1609a141b/packages/core/schematics/utils/tsurge/migration.ts#L19) for migrations using tsurge )
- have a "check" mode that tells the user if the codebase contains code that Angular could have migrated. This would make it easier to implement a "check" CI step that prevents making changes that should have been migrated

Hopefully, this should reduce the cognitive load for users.

It would be nice also to consider Nx compatibility. Right now, Nx does not have an "optional" migration concept so the `optional` flag is ignored.
As an additional input, here is how Nx handles migration opt-outs: https://nx.dev/recipes/tips-n-tricks/advanced-update

### Describe alternatives you've considered

Provide a more intuitive command than `ng g @angular/core: --help` to list all available migration schematics.
Maybe `ng update` could list them.

`ng update` コマンドと、`packages/core/schematics/utils/tsurge/migration.ts` で参照されている migration のエントリーポイントから始めてください。任意の migration と手動の migration が現在どのように一覧表示され、選択され、実行されるかを、`--migrate-only` フローも含めて確認してください。完了の条件は、Nx との互換性を考慮したうえで、migration の一覧表示、実行、プロンプト表示、確認について合意された統一的なエクスペリエンスが実現されることです。

索引モデルが issue の本文から書いたものです。