Resolve Java 21 FTBFS

This MR fixes https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1052589

Note: alternative is to provide a new upstream release (2.0.0AM27) that depends on BouncyCastle for the certificate operations rather than on internal sun.* API.

Changes:

• use reflection to workaround internal API breakage between Java 17 and 21.
• add missing --add-opens for sun.security.x509 (crash on startup)

Testing:

• build in sid chroot
• build in default Java 21 sid chroot
• installed and started with default settings in default Java 21 sid vm and default Java 17 vm
• ran apacheds core tests (added <argLine>-Djava.awt.headless=true -Xmx256m --add-opens=java.base/sun.security.util=ALL-UNNAMED --add-opens=java.base/sun.security.x509=ALL-UNNAMED</argLine> to surefire plugin configuration and set -Dcheckstyle.skip=true)