Reference doc generation is not compatible with Java 21

When generating reference doc using the command with Java 21:
```
./docs/src/main/asciidoc/ghpages.sh --version v4.8.4 --destination . --build
```
I got the following error message:
```
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-antrun-plugin:3.0.0:run (readme) on project spring-cloud-gcp-docs: An Ant BuildException has occured: java.lang.UnsupportedOperationException: The Security Manager is deprecated and will be removed in a future release
[ERROR] around Ant part ...... @ 7:132 in /Users/joewa/Documents/workspace/spring-cloud-gcp/docs/target/antrun/build-main.xml
```

When I changed to Java 17, the command finished successfully.

Start with docs/src/main/asciidoc/ghpages.sh and the reference-documentation generation command shown in the issue. Inspect the Maven antrun invocation involving org.springframework.cloud.internal.asciidoctor.ReadmeMain, then reproduce with Java 21 and compare with Java 17. Done means the documented generation command completes successfully on Java 21 without the reported Security Manager error.

Written by the indexing model from the issue text.