ci: automate Gradle and Docker image updates (#7464)

## Summary

- Add a scheduled GitHub Actions workflow that checks for the latest
stable Gradle release.
- Update the Gradle wrapper and pinned Gradle Docker build images
automatically.
- Open an automated pull request when an update is available.
- Update the developer prerequisites to Node.js 22+ and Gradle 9.0+.

## Details

The workflow runs every Monday at 03:00 UTC and can also be triggered
manually. It:

1. Resolves the latest stable Gradle version.
2. Finds the matching `gradle:<version>-jdk25` Docker image digest.
3. Updates the Gradle wrapper and Dockerfiles.
4. Verifies the resolved wrapper version and checks the resulting diff.
5. Creates or updates an automated dependency pull request.

The current wrapper and Docker image changes are included as the initial
update generated by this workflow.

## Testing

- Verified the generated changes with `git diff --check`.
- The workflow validates the wrapper version before opening the
automated pull request.

DeveloperGuide.md

@@ -46,8 +46,8 @@ This guide focuses on developing for Stirling 2.0, including both the React fron
 - Docker
 - Git
 - Java JDK 25
-- Node.js 18+ and npm (required for frontend development)
-- Gradle 7.0 or later (Included within the repo)
+- Node.js 22+ and npm (required for frontend development)
+- Gradle 9.0 or later (Included within the repo)
 - [uv](https://docs.astral.sh/uv/) — Python package manager (required for engine development)
 - Rust and Cargo (required for Tauri desktop app development)
 - Tauri CLI (install with `cargo install tauri-cli`)