Merge pull request #271 from http4s/update/scala-library-2.12.20

Update scala-library to 2.12.20

Author: danicheg

Diff for: build.sbt

@@ -21,7 +21,7 @@ lazy val docs = project
   .enablePlugins(Http4sOrgSitePlugin)
 
 ThisBuild / tlBaseVersion := "0.17"
-ThisBuild / crossScalaVersions := Seq("2.12.19")
+ThisBuild / crossScalaVersions := Seq("2.12.20")
 ThisBuild / developers := List(
   Developer(
     "rossabaker",