progressr fails to load on R<4

As seen in GHA (progressr 1.0.0):

https://github.com/Rdatatable/data.table/actions/runs/30377527575/job/90336903587#step:11:1962

```
** testing if installed package can be loaded from temporary location
Error: Error: package or namespace load failed for ‘progressr’:
.onLoad failed in loadNamespace() for 'progressr', details:
call: globalCallingHandlers()
error: register_global_progression_handler() requires R (>= 4.0.0)
```

I poked around briefly at the source code and couldn't figure out where `.onLoad()` makes the call, but it looks like there's supposed to be an optional functionality that is being triggered unconditionally.

Start with the linked GitHub Actions log and trace the package's .onLoad() path to the call to globalCallingHandlers() and register_global_progression_handler(). Reproduce installation and loading under R <4; done when progressr loads successfully without unconditionally invoking functionality unavailable in that R version.

Written by the indexing model from the issue text.