Start 6.0 version

* Upgrade to Java 17, SF-6.0, Gradle 7.2
* Upgrade to Jakarta dependencies and respective namespaces
* Fix some tests for Java 17 compatibility
* Fix wrong Javadocs
* Add some missed Javadocs
* Fix more `jakarta` namespace
* Fix WS & XML modules to use Jakarta EE
* `--add-opens` in some modules for their reflection-based tests
* Disable Kafka tests which does not work on Windows; see Apache Kafka `3.0.1`
* Upgrade to JUnit `5.8.1`
* Migrate JMS tests to Artemis
* Remove RMI module as it was deprecated before
* Fix `pr-build-workflow.yml` for Java 17
* Fix JavaDocs warnings using `Xdoclint:syntax` per module, not in the top-level `api` task
* Move docs for version `6.0`

src/checkstyle/checkstyle.xml

@@ -103,7 +103,7 @@
 			<property name="processJavadoc" value="true"/>
 		</module>
 		<module name="ImportOrder">
-			<property name="groups" value="java,/^javax?\./,org,org.springframework,*"/>
+			<property name="groups" value="java,/^javax?\./,/^jakarta?\./,org,org.springframework,*"/>
 			<property name="ordered" value="true"/>
 			<property name="separated" value="true"/>
 			<property name="option" value="top"/>