Delete or update references to Debian 8 Jessie (#8065)

* Delete or update references to Debian 8 Jessie

* Don't delete oldest constraints from Jessie, but document in comments.

* Update tools/oldest_constraints.txt

Co-authored-by: ohemorange <ebportnoy@gmail.com>

Co-authored-by: Brad Warren <bmw@users.noreply.github.com>
Co-authored-by: ohemorange <ebportnoy@gmail.com>

certbot/docs/install.rst

@@ -250,9 +250,6 @@ through a command like:
 They can be installed by running the same installation command above but
 replacing ``certbot`` with the name of the desired package.
 
-There are no Certbot packages available for Debian Jessie and Jessie users
-should instead use certbot-auto_.
-
 **Ubuntu**
 
 If you run Ubuntu Trusty, Xenial, or Bionic, certbot is available through the official PPA,

certbot/docs/using.rst

@@ -631,7 +631,6 @@ systemd timers (`systemctl list-timers`).
    :header: "Distribution Name", "Distribution Version", "Automation Method"
 
    "CentOS", "EPEL 7", "systemd"
-   "Debian", "jessie", "cron, systemd"
    "Debian", "stretch", "cron, systemd"
    "Debian", "testing/sid", "cron, systemd"
    "Fedora", "26", "systemd"