Update Scala version to 3.1.3

Author: Kordyjan

README.md

@@ -21,7 +21,7 @@ Any version number that starts with `3.` is automatically recognized as Scala 3;
 you don't need to set up anything:
 
 ```scala
-def scalaVersion = "3.1.2"
+def scalaVersion = "3.1.3"
 ```
 
 #### Nightly builds

build.sc

@@ -2,6 +2,6 @@ import mill._, scalalib._
 
 object root extends SbtModule {
   def millSourcePath = ammonite.ops.pwd
-  def scalaVersion = "3.1.2"
+  def scalaVersion = "3.1.3"
   def publishVersion = "0.1.0"
 }