Fix linking to JDK 21 from the Guava Javadoc.

Currently, we link to JDK 8 instead. It's a long story:

I had moved us to 21 in cl/696975523, but that got undone by cl/711476575: The latter change reinstated a flag (`--no-module-directories`) from cl/413922237 that had been accidentally circumvented by cl/655647768. (cl/655647768 changed the VM that we use to run _Maven itself_, and our Javadoc configuration was wrongly triggered by the version of _that_ VM, rather than the version used to run Javadoc.) That flag, which worked around a bug whose fixed has conveniently [been backported](https://bugs.openjdk.org/browse/JDK-8215291?focusedId=14506100&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-14506100), somehow also caused [a warning/error](#6790 (comment)):

```
[WARNING] warning: URL https://docs.oracle.com/javase/21/docs/api/element-list was redirected to https://docs.oracle.com/en/java/javase/21/docs/api/index.html -- Update the command-line options to suppress this warning.
```

In the cases in which it's only a warning (as when using Javadoc 21, as opposed to Javadoc 23), it is also a sign that linking to JDK 21 failed. Javadoc apparently then falls back to linking to JDK 8. (I'd say that the linking to Java 8 (rather than just failing to link altogether) is a result of [JDK-8216497](https://bugs.openjdk.org/browse/JDK-8216497) except that that change seems to have been new in Javadoc 16.)

The fix is to change the link to include the `/en/java/` segments. This is a change relative to where Java _8_ Javadoc lives to this day. (Since Java 8 Javadoc is still at the old path, I did _not_ touch a similar link in _Caliper_, which links to Java 8.)

(I suspect that we could now upgrade to Javadoc 23 if we wanted. I think we could already _link_ to a newer JDK version if we wanted. I don't think I thought very hard about picking 21 for that.)

We do still see another warning, but _shrug_:

```
[WARNING] warning: The code being documented uses packages in the unnamed module, but the packages defined in https://docs.oracle.com/en/java/javase/21/docs/api/ are in named modules.
```

Also: Bump `maven-javadoc-plugin`, and remove a now-unused Maven property.

Also, in Truth: Prevent Truth's JDK linking from breaking similarly when we upgrade the version of Javadoc that we use to generate its docs. And remove its attempted `--no-module-directories` configuration, too: That flag is no longer necessary with recent Javadoc 11 versions, and it turns out that it hasn't actually been used since cl/509829752....

RELNOTES=n/a
PiperOrigin-RevId: 711811272

android/guava/pom.xml

@@ -159,7 +159,7 @@
             </offlineLink>
           </offlineLinks>
           <links>
-            <link>https://docs.oracle.com/javase/21/docs/api/</link>
+            <link>https://docs.oracle.com/en/java/javase/21/docs/api/</link>
             <link>https://errorprone.info/api/latest/</link>
             <link>https://jspecify.dev/docs/api/</link>
           </links>

android/pom.xml

@@ -23,8 +23,6 @@
     <jspecify.version>1.0.0</jspecify.version>
     <errorprone.version>2.36.0</errorprone.version>
     <j2objc.version>3.0.0</j2objc.version>
-    <!-- Empty for all JDKs but 9-12 -->
-    <maven-javadoc-plugin.additionalJOptions></maven-javadoc-plugin.additionalJOptions>
     <project.build.outputTimestamp>2024-01-02T00:00:00Z</project.build.outputTimestamp>
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
     <!--
@@ -328,7 +326,7 @@
         </plugin>
         <plugin>
           <artifactId>maven-javadoc-plugin</artifactId>
-          <version>3.11.1</version>
+          <version>3.11.2</version>
           <configuration>
             <jdkToolchain>
               <version>21</version>

guava/pom.xml

@@ -159,7 +159,7 @@
             </offlineLink>
           </offlineLinks>
           <links>
-            <link>https://docs.oracle.com/javase/21/docs/api/</link>
+            <link>https://docs.oracle.com/en/java/javase/21/docs/api/</link>
             <link>https://errorprone.info/api/latest/</link>
             <link>https://jspecify.dev/docs/api/</link>
           </links>

pom.xml

@@ -23,8 +23,6 @@
     <jspecify.version>1.0.0</jspecify.version>
     <errorprone.version>2.36.0</errorprone.version>
     <j2objc.version>3.0.0</j2objc.version>
-    <!-- Empty for all JDKs but 9-12 -->
-    <maven-javadoc-plugin.additionalJOptions></maven-javadoc-plugin.additionalJOptions>
     <project.build.outputTimestamp>2024-01-02T00:00:00Z</project.build.outputTimestamp>
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
     <!--
@@ -323,7 +321,7 @@
         </plugin>
         <plugin>
           <artifactId>maven-javadoc-plugin</artifactId>
-          <version>3.11.1</version>
+          <version>3.11.2</version>
           <configuration>
             <jdkToolchain>
               <version>21</version>