Upgrade nb-javac to JDK 26b26

[mbien]

Upgrades the javac integration to nb-javac based on JDK 26b26.

This PR is based on a copy of the valhalla javac experiment #8995 but swaps the nb-javac dependency against staged nb-javac 26b26 based on https://github.com/lahodaj/nb-javac/commits/upgrade-jdk26/. It will be later updated to a nb-javac release from maven central.

manual testing:

• indexer smoke test
• check https://github.com/orgs/apache/projects/536/views/1 to verify/link issues

closes #4470
closes #6822
closes #8722

[mbien]

java.refactoring / EncapsulateFieldsTest.test219140() failure is interesting

the (unrelated) refactoring generates:

 private final transient_or_acc_varargs PropertyChangeSupport propertyChangeSupport = new java.beans.PropertyChangeSupport(this);

instead of:

 private final transient PropertyChangeSupport propertyChangeSupport = new java.beans.PropertyChangeSupport(this);

sounds like a change in a toString() of a modifier?

edit: fixed