Update scala-library to 2.13.15

Author: scala-steward-bot

build.sbt

@@ -1,6 +1,6 @@
 name := "gitbucket-notifications-plugin"
 organization := "io.github.gitbucket"
 version := "1.11.0"
-scalaVersion := "2.13.14"
+scalaVersion := "2.13.15"
 gitbucketVersion := "4.41.0"
 scalacOptions := Seq("-deprecation", "-language:postfixOps", "-feature")