PopenExecutor constructs a "cd '<dir>'; ..." for the chdir: option in
spawn/system/IO.popen; the directory path was embedded in single quotes
without escaping, allowing single-quote breakout and arbitrary command
injection:

  system('echo hello', chdir: "x'; touch /tmp/pwned; echo '")

MRI uses the POSIX chdir syscall directly and is not affected

Official policy for AI use when submitting PRs to JRuby is:

* You must own your changes and be able to answer questions about
  them.
* You must be able to assert copyright over those changes to help
  us avoid infringing on other projects' licenses.

Update PR template for AI use + minor tweaks

* Unsupported configured versions fall back on system default
* Unsupported configured and default versions fall back on 21

All combinations are tested.

Make JVM bytecode version detection more robust

[fix] @JRubyMethod error with required>=1 and var-args

[fix] escape shell metachars in chdir: to prevent injection

This is in advance of trying to create an abstract RubyString with
many sub-implementations, as was attempted in #9369. The
same deprecations were done for 10.1 in #9371.

See #9457

Similar to #9489 and #9474, deprecate all
public RubyArray constructors to support making it abstract in
10.1.

See #9457

Deprecate public RubyArray constructors

Deprecate all public String constructors