drop sbt 0.13 support (#67)

Co-authored-by: William Cheng <wing328hk@gmail.com>

Author: xuwei-k

build.sbt

@@ -11,7 +11,7 @@ lazy val `sbt-openapi-generator` = (project in file("."))
   .settings(
     scalaVersion := "2.12.20",
     crossScalaVersions := Seq(scalaVersion.value, "3.7.2"),
-    crossSbtVersions := List("0.13.17", "1.3.10"),
+    crossSbtVersions := List("1.3.10"),
     sbtPlugin := true,
 
     inThisBuild(List(