Fix Javadoc generation

javadoc is able to create external links to libraries documented with
"mismatching" modularity, like for JUnit Jupiter, which offers modular
documentation.
However, a warning is generated when the mismatch happens, and this is
a problem when using the `-Werror` option to treat warnings as errors.

To circumvent this problem without dropping the `-Werror` option,
Java 18 added the `--link-modularity-mismatch=(warn|info)` command
line option to specify how external documentation with wrong
modularity should be reported.

See:
* https://docs.oracle.com/en/java/javase/18/docs/specs/man/javadoc.html#standard-options-for-the-standard-doclet
* https://bugs.openjdk.org/browse/JDK-8274639
* spring-projects/spring-framework#27497

pom.xml

@@ -100,6 +100,8 @@
           <artifactId>maven-javadoc-plugin</artifactId>
           <version>3.8.0</version>
           <configuration>
+            <!-- https://bugs.openjdk.org/browse/JDK-8274639 -->
+            <additionalOptions>--link-modularity-mismatch=info</additionalOptions>
             <failOnWarnings>true</failOnWarnings>
             <links>
               <link>https://javadoc.io/static/com.google.jimfs/jimfs/${jimfs.version}/</link>