[FEATURE REQ] Improve Java devcontainer setup for contributors

[cm-collins]

Is your feature request related to a problem? Please describe.

The current devcontainer configuration can be improved for contributors using GitHub Codespaces or VS Code Dev Containers. Some settings use older top-level devcontainer properties, and the container setup can be made clearer for Java development by using the official Java devcontainer image and Java feature configuration.

Describe the solution you'd like

Update the repository devcontainer configuration to:

• Use the official Java 21 devcontainer image.
• Install Maven through the Dev Containers Java feature.
• Set JAVA_HOME in the container environment.
• Move VS Code settings and extensions under customizations.vscode.
• Configure the VS Code Java runtime.
• Ignore the generated .devcontainer/devcontainer-lock.json file.

Describe alternatives you've considered

An alternative would be to keep the existing devcontainer configuration unchanged. However, updating it makes the development environment easier to use for contributors opening the repository in Codespaces or VS Code Dev Containers.

Another alternative would be to pin exact tool versions. The current change keeps Maven configured through the devcontainer Java feature while preserving a simple setup.

Additional context

This is a development-environment-only improvement. It does not change SDK source code, generated clients, tests, Maven project structure, or runtime behavior of any Azure SDK package.

Validation performed locally:

java -version
mvn -version
git diff --cached --check
git check-ignore -v .devcontainer/devcontainer-lock.json