[isabelle-dev] Update to jdk-11.0.9+11 with arm64-linux

[Makarius]

With current Isabelle/99da14fa28b8 we are on jdk-11.0.9+11: this is the latest
longterm/stable release from https://adoptopenjdk.net

The corresponding release notes are here:
https://foojay.io/java-11/?quarter=102020&tab=highlights

We have some months to figure out if this is a truly stable JDK release for
our Isabelle2021 (scheduled for 15-Feb-2021).


Moreover, I have started to include arm64-linux routinely, for easy
experimentation with Raspberry Pi 4 / PI OS 4 (64 bit). The instructions in
README_REPOSITORY should work as usual (still missing are some external proof
tools).


	Makarius