Update model:core to 1.4.21

Author: scala-steward

build.sbt

@@ -9,7 +9,7 @@ val scala2_13 = "2.13.7"
 val scala2 = List(scala2_11, scala2_12, scala2_13)
 val scala3 = List("3.1.0")
 
-val sttpModelVersion = "1.4.20"
+val sttpModelVersion = "1.4.21"
 
 val scalaTestVersion = "3.2.10"
 val zio1Version = "1.0.13"