disable old deb.leap.se repo, direct to newer stuff
I recently discovered that the riseup help pages were still recommending that people could install debian packages from the deb.leap.se apt repo: https://riseup.net/en/vpn/linux#package-installation
The packages there are 0.19.2 based and timestamped 2019-09-10.
Also I see in #466 that there are proper packages in debian now, although just testing/unstable so far, hopefully there will be stable backports soon too.
I will work on getting the riseup help pages updated. But I think it would be a good idea to disable the old repo in such a way that anyone still using it gets a clear error and is instructed to switch to either the snap or the packages in the Debian archive. There might be reasons to keep the old packages around for history's sake, but they should be moved aside to prevent old configs or anyone installing from old instructions from automatically getting them.