Update http4s-blaze-client, http4s-circe to 0.21.6 (#15)

build.sbt

@@ -12,7 +12,7 @@ lazy val V = new {
   val cats: String      = "2.1.1"
   val circe: String     = "0.13.0"
   val classutil: String = "1.5.1"
-  val http4s: String    = "0.21.5"
+  val http4s: String    = "0.21.6"
   val scala: String     = "2.13.2"
   val scalatest: String = "3.2.0"
 }