mistriggered "error: warnings found and -Werror specified" for java warnings

[Matthias Klose]

jdk-15+20 fails to build with

* For target support_test_failure_handler_classes__the.BUILD_FAILURE_HANDLER_batch:
/packages/openjdk/15/openjdk-15-15~20/test/failure_handler/src/share/classes/jdk/test/failurehandler/jtreg/GatherDiagnosticInfoObserver.java:136:
warning: [deprecation] finishedTesting() in Observer has been deprecated
    public void finishedTesting() {
                ^
error: warnings found and -Werror specified
1 error
1 warning


Apparently --disable-warnings-as-errors only has an effect on C/C++ files,
however the build diagnostics trigger on java warnings as well, and apparently
-Werror is hard-coded in various places for java options. Should the
documentation for this configure option be clarified, or should it trigger for
java warnings as well?

Matthias