Upgrade tor to a version that still has security support

While checking something else, I noticed that the version of tor that's in Stretch, which we run on a number of our systems, is not covered by Debian's security support anymore:

https://salsa.debian.org/debian/debian-security-support/blob/master/security-support-ended.deb9

I believe the recommended way to fix this is to install tor (and presumably its companion tor-geoipdb package) from stretch-backports.