Dev: Make getInsertID() more consistent with save()

### PHP Version

8.2, 8.3

### CodeIgniter4 Version

latest

### CodeIgniter4 Installation Method

Composer (using `codeigniter4/appstarter`)

### Which operating systems have you tested for this bug?

macOS, Linux

### Which server did you use?

fpm-fcgi

### Database

MaraiaDB

### What happened?

Not exactly a bug, but not really a FR either. See this forum thread: https://forum.codeigniter.com/showthread.php?tid=91800&pid=421782#pid421782

Essentially I'm offering to PR a change to `\System\Model::update()` that would set an instance's `$insertID` property the same way `\System\Model::insert()` does so that the behaviour of `save()` is more consistent.

Currently `\System\Model::getInsertID()` only returns a value if a `save()` does an insert. The logic needed to handle that works against the elegance provided by `save()`.

### Steps to Reproduce

Use `\System\Model::save()`

### Expected Output

Best case, `\System\Model::getInsertID()` will return the "affected ID" after `\System\Model::save()` regardless of which action occurred.

### Anything else?

I'm stoked contribute to core, but I didn't want to spend the time on a PR if there is no desire to change this behaviour. Please advise.

I can write the update to the docs too if I'm pointed in the right direction to do so.