[#286] disable SecurityManager starting with Java 21

[bmarwell]

fixes #286

[bmarwell]

You can ignore the Maven warning for now, it will go away with the next maven-compiler-plugin release: apache/maven-compiler-plugin#202

[bmarwell]

@keeganwitt please review (cannot tag you as a reviewer)

[keeganwitt]

Do you think we should include something about -Djava.security.manager=allow? for Java 18, 19, 20 users? I thought it might be useful, but then started second-guessing it since those aren't LTS releases.

[keeganwitt]

Do you think we should include something about -Djava.security.manager=allow? for Java 18, 19, 20 users? I thought it might be useful, but then started second-guessing it since those aren't LTS releases.

Actually this applies to Java 21 users as well. I don't think it's been removed (yet), just turned off by default. This being the case, should we really be turning it off entirely?

[bmarwell]

Do you think we should include something about -Djava.security.manager=allow? for Java 18, 19, 20 users? I thought it might be useful, but then started second-guessing it since those aren't LTS releases.

Actually this applies to Java 21 users as well. I don't think it's been removed (yet), just turned off by default. This being the case, should we really be turning it off entirely?

That's true.
Other things we could do:

• guess any future version (Java 24, Java 30, whatever) and change all java21 occurrences to any future version
• For now, just print a warning that the feature will be removed (regardless of the -XX options of the user)
• and/or add the META-INF/versions/javaXX code path later

[bmarwell]

@keeganwitt this pr is flawed, it should be META-INF/versions/21, not ../java21. Will update the PR.

[keeganwitt]

For now, I'm gonna proceed with the warning. But this PR will likely prove useful down the road.