This seems to be defined in scala3/project/Build.scala at main · scala/scala3 · GitHub - is not it time to update to 2.13.14?
/** scala-library version required to compile Dotty.
*
* Both the non-bootstrapped and bootstrapped version should match, unless
* we're in the process of upgrading to a new major version of
* scala-library.
*/
def stdlibVersion(implicit mode: Mode): String = mode match {
case NonBootstrapped => "2.13.12"
case Bootstrapped => "2.13.12"
}