Add support for Scala 2.13.9 (#680)

Co-authored-by: Henri Cook <[email protected]>

Author: henricook

.github/workflows/pr-checks.yml

@@ -13,9 +13,9 @@ jobs:
           - 2.11.12
           - 2.12.15
           - 2.12.16
-          - 2.13.6
           - 2.13.7
           - 2.13.8
+          - 2.13.9
     steps:
       - uses: actions/checkout@v2
       - uses: olafurpg/setup-scala@v10

README.md

@@ -4,7 +4,7 @@ Scapegoat
 [![Codecov](https://img.shields.io/codecov/c/github/sksamuel/scapegoat)](https://codecov.io/gh/sksamuel/scapegoat)
 [<img src="https://img.shields.io/maven-central/v/com.sksamuel.scapegoat/scalac-scapegoat-plugin_2.11.12.svg?label=latest%20release%20for%202.11.12"/>](http://search.maven.org/#search%7Cga%7C1%7Ca%3A%22scalac-scapegoat-plugin_2.11.12%22)
 [<img src="https://img.shields.io/maven-central/v/com.sksamuel.scapegoat/scalac-scapegoat-plugin_2.12.16.svg?label=latest%20release%20for%202.12.16"/>](http://search.maven.org/#search%7Cga%7C1%7Ca%3A%22scalac-scapegoat-plugin_2.12.16%22)
-[<img src="https://img.shields.io/maven-central/v/com.sksamuel.scapegoat/scalac-scapegoat-plugin_2.13.8.svg?label=latest%20release%20for%202.13.8"/>](http://search.maven.org/#search%7Cga%7C1%7Ca%3A%22scalac-scapegoat-plugin_2.13.8%22)
+[<img src="https://img.shields.io/maven-central/v/com.sksamuel.scapegoat/scalac-scapegoat-plugin_2.13.9.svg?label=latest%20release%20for%202.13.9"/>](http://search.maven.org/#search%7Cga%7C1%7Ca%3A%22scalac-scapegoat-plugin_2.13.9%22)
 [![Scala Steward badge](https://img.shields.io/badge/Scala_Steward-helping-blue.svg?style=flat&logo=data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAA4AAAAQCAMAAAARSr4IAAAAVFBMVEUAAACHjojlOy5NWlrKzcYRKjGFjIbp293YycuLa3pYY2LSqql4f3pCUFTgSjNodYRmcXUsPD/NTTbjRS+2jomhgnzNc223cGvZS0HaSD0XLjbaSjElhIr+AAAAAXRSTlMAQObYZgAAAHlJREFUCNdNyosOwyAIhWHAQS1Vt7a77/3fcxxdmv0xwmckutAR1nkm4ggbyEcg/wWmlGLDAA3oL50xi6fk5ffZ3E2E3QfZDCcCN2YtbEWZt+Drc6u6rlqv7Uk0LdKqqr5rk2UCRXOk0vmQKGfc94nOJyQjouF9H/wCc9gECEYfONoAAAAASUVORK5CYII=)](https://scala-steward.org)
 
 Scapegoat is a Scala static code analyzer, what is more colloquially known as a code lint tool or linter. Scapegoat works in a similar vein to Java's [FindBugs](http://findbugs.sourceforge.net/) or [checkstyle](http://checkstyle.sourceforge.net/), or Scala's [Scalastyle](https://github.com/scalastyle/scalastyle).

build.sbt

@@ -22,8 +22,8 @@ developers := List(
   )
 )
 
-scalaVersion := "2.13.8"
-crossScalaVersions := Seq("2.11.12", "2.12.15", "2.12.16", "2.13.7", "2.13.8")
+scalaVersion := "2.13.9"
+crossScalaVersions := Seq("2.11.12", "2.12.15", "2.12.16", "2.13.7", "2.13.8", "2.13.9")
 autoScalaLibrary := false
 crossVersion := CrossVersion.full
 crossTarget := {