The `GetSlice()` function is part of cobra's [cobra.SliceValue] interface,
and duplicates the older `GetAll()` method. This patch changes our use
of the `GetAll()` method with the intent to deprecated it in future.
[cobra.SliceValue]: https://pkg.go.dev/github.com/spf13/cobra@v1.9.1#SliceValue
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>