Document update-gradle-wrappers script usage and functionality

Author: rabestro

docs/update-gradle-wrappers.md

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+### Update Gradle Wrappers Across All Exercises
+
+This repository contains many independent Gradle projects (one per exercise). 
+To update all Gradle wrappers to a specific version (e.g., to support Java 25), use the helper script below.
+
+#### Script name
+
+Suggested name: `update-gradle-wrappers` (added under `bin/`).
+
+#### Requirements
+
+- macOS/Linux shell with Bash
+- Internet access (the first update will download the target Gradle distribution)
+
+#### Usage
+
+```
+bin/update-gradle-wrappers [<gradle-version>]
+```
+
+- If omitted, the version defaults to `9.2.1`.
+- Example: update everything to Gradle 9.2.1
+
+```
+bin/update-gradle-wrappers 9.2.1
+```
+
+#### What the script does
+
+1. Uses the fixed seed at `exercises/gradle` and runs:
+   ```
+   ./gradlew wrapper --gradle-version <version>
+   ```
+   This produces the correct `gradle-wrapper.jar` and `gradle-wrapper.properties` for the requested version.
+2. Copies the wrapper files from the seed to every exercise’s `gradle/wrapper/` directory
+
+This aligns with the requested approach: update via Gradle, and ensure consistency by copying the wrapper files everywhere.
+
+#### Notes
+
+- The script uses `exercises/gradle` as the single source of truth (seed) and does not run Gradle in each exercise; it only copies the wrapper files.
+- The operation can take time on the first run due to Gradle downloads.
+
+#### Troubleshooting
+
+- If you see permission issues, make the script executable:
+  ```
+  chmod +x bin/update-gradle-wrappers
+  ```