samples: net: big_http_download: Update URL for TLS download

The old link has expired, the file is no longer avaiable to download.

Update the download link, which now leads to a file in Ubuntu
repositories for the recent LTS release, hopefully making the link valid
for longer period.

As the server ceritficate is signed by a different root CA, update the
root CA as well.

Finally, cleanup how the URL is handled in the sample - for TLS variant
it'll always be provided with a Kconfig, hence no need to repeat it in
the sample.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>