… and upgrading dev dependency pins (#4802)

# Description of Changes

This pull request updates the development requirements to address
security vulnerabilities and improve dependency management. The most
important changes include switching the `brotli` dependency to a
specific commit for CVE mitigation, and upgrading the `filelock`
package.

**Security and Dependency Management Updates:**

* Pinned the `brotli` package to a specific commit from the official
GitHub repository in both `requirements_dev.in` and
`requirements_dev.txt` to mitigate CVE-2025-6176. This replaces the
previous PyPI version and removes hash checks, ensuring a secure and
up-to-date version is used.
[[1]](diffhunk://#diff-8ea1287e3b069fa12ef70955fbeffacf656f7b409d13c8f52d7506ac7eb383abL1-R9)
[[2]](diffhunk://#diff-5d7664bae1e6bf71ccbc8e524e6777e3a05e5899ae64cbdccabe36eccd15520dL7-R13)
* Upgraded the `filelock` package from version 3.19.1 to 3.20.0 in
`requirements_dev.txt`, updating hashes accordingly.

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## 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/devGuide/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)

### 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 tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md#6-testing)
for more details.