[isabelle-dev] scala-2.13.4

[Makarius]

With Isabelle/ca17e9ebfdf1 both isabelle-dev and isabelle-release are using
current scala-2.13.4; before it was scala-2.12.12.

There are significant differences between 2.12.x and 2.13.x. For us the main
impact is a change in ArraySeq, which has more static types in 2.13.x.

See also Isabelle/ca450d902198 (no use of ArraySeq in HOL-Imperative_HOL) and
AFP/4ee343798f76 (use of dynamically typed ArraySeq[AnyRef] in Collections).


Originally, I did not plan to have Isabelle2021 on scala-2.13.x for the sake
of Isabelle/MMT. But now it turned out that the MMT project was unusally quick
in adopting current Scala.


	Makarius