Update Java devcontainer configuration

[cm-collins]

Description

Updates the Java devcontainer configuration to improve the contributor experience when using GitHub Codespaces or VS Code Dev Containers.

This PR:

• Switches the devcontainer to the official Java 21 devcontainer image.
• Installs Maven through the Dev Containers Java feature.
• Sets JAVA_HOME for the container environment.
• Moves VS Code settings and extension recommendations under customizations.vscode.
• Adds the VS Code Java extension pack.
• Ignores the generated .devcontainer/devcontainer-lock.json file.

This is a development-environment-only change. It does not modify SDK source code, generated clients, public APIs, tests, package versions, or build logic.

Fixes #49617

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes. N/A: devcontainer-only contributor tooling change.
• I have read the contribution guidelines.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message.

Testing Guidelines

• Pull request includes test coverage for the included changes. N/A: no SDK runtime code changed; validation was performed against the devcontainer configuration.

Validation

Validated locally in the devcontainer:

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

[github-actions]

Thank you for your contribution @cm-collins! We will review the pull request and get back to you soon.

[Copilot]

Pull request overview

Updates the repository’s Dev Container configuration to use the official Java 21 devcontainer image and modern Dev Containers/VS Code customization structure, improving the setup experience in GitHub Codespaces and VS Code Dev Containers.

Changes:

• Switches to mcr.microsoft.com/devcontainers/java:3-21-bookworm and installs Maven via the Dev Containers Java feature.
• Moves VS Code settings and extension recommendations under customizations.vscode, and adds the Java extension pack.
• Ignores the generated .devcontainer/devcontainer-lock.json file.

Reviewed changes

Copilot reviewed 1 out of 2 changed files in this pull request and generated 1 comment.

File	Description
.gitignore	Ignores the generated devcontainer lock file to avoid accidental commits.
.devcontainer/devcontainer.json	Migrates to the Java 21 devcontainer image, configures Maven/JAVA_HOME, and relocates VS Code settings/extensions to customizations.vscode.