Link to Java 21 Javadoc instead of Java 9.

[copybara-service]

Link to Java 21 Javadoc instead of Java 9.

(Compare cl/696632561 for Truth. I verified that the Truth change worked in post-submit CI, and (as you'll see) I did some work to verify that this Guava change works in local testing.)

This CL addresses the "linking" part of #6790 but not the "inheriting" part.

I was having trouble getting this to work with the JDK in the <linkoffline> section, so I moved it to the <link> section. (But not before first accidentally creating a second <link> section, which Maven seems to have chosen to ignore without error. See also my other complaints in a comment in the actual pom.xml.) Fortunately, we don't need to use <linkoffline> for the JDK anymore because we no longer need to have the JDK in the same section as JSR-305 now that javax.annotation isn't part of the JDK anymore (and thus there's no longer a conflict about whether the JDK or JSR-305 gets to "claim" it and thus there's no need to resolve it through ordering, which we can control only within sections, not across sections).

Somewhere in the course of working through that, I upgraded maven-javadoc-plugin to 3.11.1. That may or may not have turned out to be necessary, but it's probably more likely to help than to hurt.

RELNOTES=n/a