more recent OCaml and GHC stack: better support for Apple Silicon;

--- a/etc/settings	Sun Apr 25 22:33:53 2021 +0200
+++ b/etc/settings	Wed Apr 28 12:24:39 2021 +0200
@@ -152,7 +152,7 @@
 
 ISABELLE_OPAM_ROOT="$USER_HOME/.opam"
 
-ISABELLE_OCAML_VERSION="ocaml-base-compiler.4.07.0"
+ISABELLE_OCAML_VERSION="ocaml-base-compiler.4.12.0"
 
 
 ###
@@ -161,9 +161,9 @@
 
 ISABELLE_STACK_ROOT="$USER_HOME/.stack"
 
-ISABELLE_STACK_RESOLVER="lts-16.31"
+ISABELLE_STACK_RESOLVER="lts-17.10"
 
-ISABELLE_GHC_VERSION="ghc-8.8.4"
+ISABELLE_GHC_VERSION="ghc-8.10.4"
 
 
 ###