Fix obsolete debian watch

Summary:
The debian watch file was pointing to a non existing page, and used an
obsolete version number. This diff updates this file based on the
documentation from https://wiki.debian.org/debian/watch. The github
pattern is adapted to only match the source archive, since we don't want
uscan to download a binary from another architecture.

Test Plan:

cd /contrib
uscan --debug

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7732