Also make sure our documentation for upgrading is reasonable for
Stretch => Buster.
Our reasoning for deprecating support for these releases is as follows:
* Ubuntu 16.04 Xenial reached desktop EOL last year; and will reach
EOL on the server in about a year.
* Debian Stretch will each EOL in 2020 (the precise date is unclear in
Debian's documentation, but based on past precedent it's in the next
few months, perhaps July 2020).
https://wiki.debian.org/DebianReleases#Production_Releases
* Both Ubuntu 16.04 and Debian Stretch use Python 3.5 as the system
Python, which will reach EOL in September 2020 (and we're already
seeing various third-party dependencies that we use drop support for
them).
* While there is LTS support for these older releases, it's not clear it's
going to be worth the added engineering effort for us to maintain EOL
releases of the base OSes that we support.
* We (now) have clear upgrade instructions for moving to Debian Buster
and Ubuntu 18.04.
This also rewrites the text to better explain what's happening. It's
likely further polish would be valuable, but that's true for the whole
"Troubleshooting" page.
This block of text was misplaced when we split the long
maintain-secure-update; article; we want it to be easy to find by
folks who are looking into error emails Zulip is sending.
Merges the "Upgrades" section from production/maintain-secure-upgrade.md
with production/modifying-zulip.md.
Contains significant textual changes by tabbott to read more clearly.
Tim Abbott
Mateusz Mandera
David Rosa