Upgrade ch.qos.logback dependency

Upgrade ch.qos.logback to version 1.2.9 to fix a JNDI-related vulnerability.

https://logback.qos.ch/news.html

logback-android is unaffected as it doesn't support JNDI. How sensible.

https://github.com/tony19/logback-android/issues/230

changed milestone to %Mailbox

added Mailbox P: high Security Sponsor 6 T: task labels

made the issue visible to everyone

added Mailbox app label

added S: in progress label

assigned to @sebkur

added Weight: 1 label

added S: in code review label and removed S: in progress label

mentioned in merge request !60 (merged)

mentioned in commit 96df7155

closed with merge request !60 (merged)

added R: fixed label and removed S: in code review label