RequestProcessor warning fixes and minor update

 - use SortedSet#removeFirst instead of getFirst()+remove()
 - remove ThreadDeath and AccessController.doPrivileged() usage
 - convert Maps of type WeakHashMap to Set where applicable
 - generics rawtype warning fixes, diamonds, overrides and similar
   minor renovations
 - javadoc typos

Author: mbien

platform/openide.util/nbproject/project.properties

@@ -14,7 +14,7 @@
 # KIND, either express or implied.  See the License for the
 # specific language governing permissions and limitations
 # under the License.
-javac.release=17
+javac.release=21
 javac.compilerargs=-Xlint -Xlint:-serial
 javadoc.arch=${basedir}/arch.xml
 module.jar.dir=lib