Update sbt-ci-release to 1.6.0 (#1719)

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

Author: scala-steward

project/plugins.sbt

@@ -18,7 +18,7 @@ addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.5.2")
 
 addSbtPlugin("com.thesamet" % "sbt-protoc" % "1.0.7")
 
-addSbtPlugin("com.github.sbt" % "sbt-ci-release" % "1.5.12")
+addSbtPlugin("com.github.sbt" % "sbt-ci-release" % "1.6.0")
 
 addSbtPlugin("com.thesamet" % "sbt-protoc-gen-project" % "0.1.8")