GHA workflows - are we doing too much CI testing?

The Julia track currently tests 3 versions of Julia against 3 OS's, which seems unusual compared with similar tracks. This is the relevant part of `exercise-tests.yml`:

```yaml
test:
name: Julia ${{ matrix.julia-version }} - ${{ matrix.os }}
runs-on: ${{ matrix.os }}
strategy:
fail-fast: false
matrix:
julia-version: ["1.6", "1", nightly]
os: [ubuntu-22.04, windows-2022, macos-14]
exclude:
- julia-version: 1.6
os: macos-14
```

Python and R both seem fine with just using ubuntu-22.04. Even C# uses only ubuntu-20.04, despite its Windows heritage.

I'm particularly keen to get rid of the Windows/Julia 1.6 combination, which is glacially slow (15-20 minutes, compared with <5 min for all the others). That should go away in any case later this year, when 1.6 is due to be dropped as the LTS version, but maybe Windows and Mac are now inappropriate?

I don't know the history of this choice, but perhaps it made more sense when Julia was less mature and stable, so there could be important differences between platforms.

Thoughts?