Add compatibility with Java 19

[Don-vip]

Followup of #21005

First ​​​​EA builds of Java 19 are available.

Followup: #22543.

[taylor.smock]

Right now, we aren't building/testing against Java 19 EA. See #22037 ("Projekt Java-EarlyAccess-Update-JDK" fails downloading Java with 404).

With that being said, I've been able to (at least) build on Java 19 in a docker container.

$ docker run -it --rm ubuntu:latest /bin/bash
# apt-get update && \
  apt-get install -y unzip zip && \
  curl -s "https://get.sdkman.io" | bash && \
  source "/root/.sdkman/bin/sdkman-init.sh" && \
  echo "sdkman_auto_answer=true" > ~/.sdkman/etc/config && \
  sdk install java $(sdk list java | awk -F'|' '/19/ && /open/ {print $6}' | head -n1) && \
  svn co https://josm.openstreetmap.de/osmsvn/applications/editors/josm && \
  cd josm/core && \
  ant clean dist && \
  ant test

EDIT: Tests are passing under Java 19 as of right now (r18439).