CI is broken on latest JRuby

[eregon]

cc @headius
e.g. https://github.com/ruby/strscan/actions/runs/7444965132/job/20252330495?pr=74
fails with:

Run bundle exec rake compile
mkdir -p lib
mkdir -p tmp/java/strscan
javac -target 1.8 -source 1.8 -Xlint -d tmp/java/strscan -cp /home/runner/.rubies/jruby-head/lib/jruby.jar ext/jruby/org/jruby/ext/strscan/RubyStringScanner.java ext/jruby/org/jruby/ext/strscan/StringScannerLibrary.java
warning: [options] bootstrap class path not set in conjunction with -source 8
ext/jruby/org/jruby/ext/strscan/RubyStringScanner.java:132: error: Region is abstract; cannot be instantiated
            return new Region(beg, end);
                   ^
ext/jruby/org/jruby/ext/strscan/RubyStringScanner.java:138: error: cannot find symbol
            return region.beg[index];
                         ^
  symbol:   variable beg
  location: variable region of type Region
ext/jruby/org/jruby/ext/strscan/RubyStringScanner.java:144: error: cannot find symbol
            return region.end[index];
                         ^
  symbol:   variable end
  location: variable region of type Region
ext/jruby/org/jruby/ext/strscan/RubyStringScanner.java:150: error: cannot find symbol
            return region.beg[index] = value;
                         ^
  symbol:   variable beg
  location: variable region of type Region
ext/jruby/org/jruby/ext/strscan/RubyStringScanner.java:156: error: cannot find symbol
            return region.end[index] = value;
                         ^
  symbol:   variable end
  location: variable region of type Region
ext/jruby/org/jruby/ext/strscan/RubyStringScanner.java:162: error: cannot find symbol
            return region.numRegs;
                         ^
  symbol:   variable numRegs
  location: variable region of type Region
ext/jruby/org/jruby/ext/strscan/RubyStringScanner.java:894: warning: [rawtypes] found raw type: RubyArray
        RubyArray newAry;
        ^
  missing type arguments for generic class RubyArray<T>
  where T is a type-variable:
    T extends IRubyObject declared in class RubyArray
ext/jruby/org/jruby/ext/strscan/RubyStringScanner.java:916: warning: [rawtypes] found raw type: RubyArray
        RubyArray newAry;
        ^
  missing type arguments for generic class RubyArray<T>
  where T is a type-variable:
    T extends IRubyObject declared in class RubyArray
ext/jruby/org/jruby/ext/strscan/RubyStringScanner.java:69: warning: [serial] serializable class RubyStringScanner has no definition of serialVersionUID
public class RubyStringScanner extends RubyObject {
       ^
6 errors
4 warnings
rake aborted!

(from #74)
master uses 9.4.1.0 as a workaround since 5aae8bf

[olleolleolle]

Fixed by #76