Run javadoc workflow under JDK 21 (langchain4j#1927)

Build javadoc for all modules

## Issue
Closes #

## Change

- Run javadoc workflow under JDK 21; Generate javadoc for all modules


## General checklist

- [x] There are no breaking changes
- [ ] I have added unit and integration tests for my change
- [ ] I have manually run all the unit and integration tests in the
module I have added/changed, and they are all green
- [ ] I have manually run all the unit and integration tests in the
[core](https://github.com/langchain4j/langchain4j/tree/main/langchain4j-core)
and
[main](https://github.com/langchain4j/langchain4j/tree/main/langchain4j)
modules, and they are all green
<!-- Before adding documentation and example(s) (below), please wait
until the PR is reviewed and approved. -->
- [ ] I have added/updated the
[documentation](https://github.com/langchain4j/langchain4j/tree/main/docs/docs)
- [ ] I have added an example in the [examples
repo](https://github.com/langchain4j/langchain4j-examples) (only for
"big" features)
- [ ] I have added/updated [Spring Boot
starter(s)](https://github.com/langchain4j/langchain4j-spring) (if
applicable)

.github/workflows/javadoc.yaml

@@ -27,16 +27,14 @@ jobs:
       - name: Check Out Source Code
         uses: actions/checkout@v2
 
-      - name: Set up JDK 17
+      - name: Set up JDK 21
         uses: actions/setup-java@v4
         with:
-          java-version: '17'
+          java-version: '21'
           distribution: 'temurin'
-
       - name: Generate Javadoc
         run: |
-          mvn javadoc:aggregate -DskipTests \
-            -pl '!code-execution-engines/langchain4j-code-execution-engine-graalvm-polyglot,!langchain4j-neo4j'
+          mvn -T4C compile javadoc:aggregate
 
       - name: Setup Pages
         uses: actions/configure-pages@v3