chore(ci): upgrade Gradle to 9.6.0 across workflows, Docker builds, and wrapper (#6790)

# Description of Changes

## What was changed

- Updated all GitHub Actions workflows using Gradle from older versions
(9.3.1 and 9.5.1) to Gradle 9.6.0.
- Updated the Gradle Wrapper distribution URL to use Gradle 9.6.0.
- Updated all Gradle-based Docker build stages to use the
`gradle:9.6.0-jdk25` image and corresponding image digest.
- Aligned CI, Docker, and local development environments on the same
Gradle version.
- Included the regenerated `gradlew` script changes produced by the
Gradle wrapper update process.

## Why the change was made

- Ensures consistent Gradle versions across local development, CI
workflows, and Docker builds.
- Takes advantage of the latest Gradle 9.6.0 improvements, fixes, and
compatibility updates.
- Reduces the risk of version mismatches causing build or deployment
inconsistencies.
- Simplifies maintenance by standardizing the build toolchain throughout
the repository.

---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have run `task check` to verify linters, typechecks, and tests
pass
- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md#7-testing)
for more details.

.github/workflows/docker-compose-tests.yml

@@ -61,7 +61,7 @@ jobs:
       - name: Setup Gradle
         uses: gradle/actions/setup-gradle@50e97c2cd7a37755bbfafc9c5b7cafaece252f6e # v6.1.0
         with:
-          gradle-version: 9.5.1
+          gradle-version: 9.6.0
           cache-disabled: true
 
       - name: Set up Docker Buildx