Reference docs should not use obsolete "junit5" links #35892
Description
Several links in the reference manual still link to https://junit.org/junit5/ which no longer exists.
The JUnit site has redirects in place; however, we should simply link to
https://junit.org/.
Similarly, the JUnit documentation no longer resides at https://junit.org/junit5/docs/current/user-guide/ but rather at https://docs.junit.org/current/user-guide/.