Set version to 0.21.0 (#133)

Author: sstone

build.gradle.kts

@@ -23,7 +23,7 @@ buildscript {
 
 allprojects {
     group = "fr.acinq.secp256k1"
-    version = "0.21.0-SNAPSHOT"
+    version = "0.21.0"
 
     repositories {
         google()