chdist: replace deprecated given/when with if/elsif/else

On Ubuntu 24.04, we see the following printed when invoking chdist:

given is deprecated at ./scripts/chdist.pl line 710. when is deprecated at ./scripts/chdist.pl line 711. when is deprecated at ./scripts/chdist.pl line 714. when is deprecated at ./scripts/chdist.pl line 717. when is deprecated at ./scripts/chdist.pl line 720. when is deprecated at ./scripts/chdist.pl line 723. when is deprecated at ./scripts/chdist.pl line 726. when is deprecated at ./scripts/chdist.pl line 729. when is deprecated at ./scripts/chdist.pl line 732. when is deprecated at ./scripts/chdist.pl line 735. when is deprecated at ./scripts/chdist.pl line 738. when is deprecated at ./scripts/chdist.pl line 741. when is deprecated at ./scripts/chdist.pl line 744. when is deprecated at ./scripts/chdist.pl line 747. when is deprecated at ./scripts/chdist.pl line 750. when is deprecated at ./scripts/chdist.pl line 753. when is deprecated at ./scripts/chdist.pl line 756. when is deprecated at ./scripts/chdist.pl line 759. when is deprecated at ./scripts/chdist.pl line 762.

So fix these by replacing the use of given/when/default with standard if/elsif/else

Signed-off-by: Alex Murray alex.murray@canonical.com