Link to Java 21 Javadoc instead of Java 7.

I had hoped that maybe this would help with the error that I saw when building with the Javadoc from JDK 21:

```
[ERROR] error: Unexpected redirection for URL https://docs.oracle.com/javase/7/docs/api/element-list to https://docs.oracle.com/javase/7/docs/api/
```

It turns out not to help with that.

However, the change does lead us to produce actual links to JDK types in e.g., http://truth.dev/StringSubject. (This sounds like it's _different_ from what I previously saw in Guava, where [_generating Javadoc with JDK 21_ (while _not_ updating the version we _link_ to)](google/guava#6790 (comment)) _also_ fixed links. I guess I should consider updating Guava to link to Java 21, too.)

Our old behavior of linking to Java 7 arguably made some sense when that was the minimum version that we supported. But we've [required Java 8 for a while now](https://github.com/google/truth/releases/tag/v1.1.4). And really, _some_ even newer version seems likely to be useful to most users, who have largely upgraded. And given that Google search is already not great at finding Javadoc at all (let alone new versions), maybe it's good for us to get into the practice of linking to newer versions to see if that helps improve their rankings??

RELNOTES=n/a
PiperOrigin-RevId: 696632561

pom.xml

@@ -247,7 +247,7 @@
               <link>https://guava.dev/releases/snapshot-jre/api/docs</link>
               <link>https://protobuf.dev/reference/java/api-docs/</link>
               <link>https://junit.org/junit4/javadoc/latest/</link>
-              <link>https://docs.oracle.com/javase/7/docs/api/</link>
+              <link>https://docs.oracle.com/javase/21/docs/api/</link>
             </links>
             <sourceFileExcludes>
               <sourceFileExclude>**/super/**/*.java</sourceFileExclude>