Update scala-library to 2.13.15 (#624)

Co-authored-by: eikek-scala-steward <[email protected]>

Author: eikek-scala-steward[bot]

build.sbt

@@ -1,7 +1,7 @@
 import com.github.sbt.git.SbtGit.GitKeys
 
 val scala212 = "2.12.20"
-val scala213 = "2.13.14"
+val scala213 = "2.13.15"
 val scala3 = "3.4.3"
 
 val updateReadme = inputKey[Unit]("Update readme")