Update scala-library to 2.13.9 (#13)

Co-authored-by: kenji yoshida <6b656e6a69@gmail.com>

Author: scala-steward-bot

build.sbt

@@ -1,5 +1,5 @@
 name := "gitbucket-helloworld-plugin"
 organization := "io.github.gitbucket"
 version := "1.0.0"
-scalaVersion := "2.13.8"
+scalaVersion := "2.13.9"
 gitbucketVersion := "4.38.2"