Update zio, zio-streams, zio-test, ... to 2.1.21 (#851)

Author: scala-steward

build.sbt

@@ -1,6 +1,6 @@
 val mainScala        = "2.13.16"
 val allScala         = Seq("3.3.6", "2.13.16", "2.12.20")
-val zioVersion       = "2.1.20"
+val zioVersion       = "2.1.21"
 val zioAwsVersion    = "7.31.52.2"
 val elasticMqVersion = "1.6.14"