[3rdparty] logback 1.2.8

This fixes remote code execution vulnerability in logback, similar to
log4shell.
See http://logback.qos.ch/news.html

Differential Revision: https://phabricator.twitter.biz/D801083

build.sbt

@@ -6,7 +6,7 @@ Global / excludeLintKeys += scalacOptions
 // All Twitter library releases are date versioned as YY.MM.patch
 val releaseVersion = "21.12.0-SNAPSHOT"
 
-val logbackVersion = "1.2.6"
+val logbackVersion = "1.2.8"
 val opencensusVersion = "0.19.1"
 val slf4jVersion = "1.7.30"