Added entry to `changelog.md` for PR fixing accumulation of confusion
matrix across jobs. As mentioned in
https://github.com/opencv/cvat/pull/7604 this is a PR where the
necessary changelog entry has been added.

### Checklist
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- [x] I submit my changes into the `develop` branch
- [x] I have created a changelog fragment <!-- see top comment in
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~- [ ] I have updated the documentation accordingly~
~- [ ] I have added tests to cover my changes~
~- [ ] I have linked related issues (see [GitHub docs](

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~- [ ] I have increased versions of npm packages if it is necessary

([cvat-canvas](https://github.com/opencv/cvat/tree/develop/cvat-canvas#versioning),

[cvat-core](https://github.com/opencv/cvat/tree/develop/cvat-core#versioning),

[cvat-data](https://github.com/opencv/cvat/tree/develop/cvat-data#versioning)
and

[cvat-ui](https://github.com/opencv/cvat/tree/develop/cvat-ui#versioning))~

### License

- [x] I submit _my code changes_ under the same [MIT License](
https://github.com/opencv/cvat/blob/develop/LICENSE) that covers the
project.
  Feel free to contact the maintainers if that's a concern.