Make `asdf update` update all plugins

### Is your feature request related to a problem? Please describe

I could not install Node.js 24. `asdf list all nodejs` stopped at 23.x, so I assumed 24 simply was not available yet.

It turned out my nodejs plugin was a outdated from March 2025, months before Node 24 was released. `asdf plugin update nodejs` fixed it in seconds. Until then I did not know I must update plugins myself.

Nothing in the output pointed there. The version list looked complete rather than months old, so the missing version read as "does not exist" instead of "your copy is stale". Meanwhile asdf update (the command one would naturally reach for) only prints a notice saying asdf is upgraded through my package manager, which sounds like there is nothing left to update locally.

### Describe the proposed solution

Reuse the `asdf update` command to update plugins, like `brew update` updates its taps and formulae.

### Describe similar `asdf` features and why they are not sufficient

`asdf plugin update --all` already does the right thing. The problem is finding it / thinking about it. `asdf update` actively leads away from it, and neither `asdf list all` nor a failed `asdf install` gives any hint about how old the version list is.

### Describe other workarounds you've considered

None

Start with the existing `asdf update` entry point and compare its behavior with `asdf plugin update --all`; also inspect how `asdf list all` and a failed `asdf install` report missing versions. Done means `asdf update` updates all plugins and its output makes that work clear, with the relevant CLI tests passing.

Written by the indexing model from the issue text.