debian/copyright: use https URLs when available

This is just to fix some noise when running `duck` and make it easier to
find out more useful fixes in the future.

This fixes the following notices from `duck`:

I: debian/copyright:954: URL: http://www.gnu.org/licenses/: INFORMATION (Certainty:possible)
   The web page at http://www.gnu.org/licenses/ works, but is also available via https://www.gnu.org/licenses/, please consider switching to HTTPS urls.

I: debian/copyright:933: URL: http://www.gnu.org/licenses/: INFORMATION (Certainty:possible)
   The web page at http://www.gnu.org/licenses/ works, but is also available via https://www.gnu.org/licenses/, please consider switching to HTTPS urls.

I: debian/copyright:108: URL: http://fsf.org/: INFORMATION (Certainty:possible)
   URL schema changed from HTTP to HTTPS during redirect(s): http://fsf.org -> https://www.fsf.org
   Please investigate and update the URL eventually, to avoid unneccesary redirects!