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.

從 `ng update` 命令以及 `packages/core/schematics/utils/tsurge/migration.ts` 中參照的 migration 入口開始。了解目前如何列出、選取和執行選用的 migration 與手動 migration，包括 `--migrate-only` 流程。完成的要求是：在考量 Nx 相容性的情況下，就列出、執行、提示和檢查 migration 形成一致且統一的體驗。

由索引模型根據 Issue 內容生成。