Bump braintree from 4.21.0 to 4.28.0

Bumps [braintree](https://github.com/braintree/braintree_python) from 4.21.0 to 4.28.0.
- [Changelog](https://github.com/braintree/braintree_python/blob/master/CHANGELOG.md)
- [Commits](braintree/braintree_python@4.21.0...4.28.0)

---
updated-dependencies:
- dependency-name: braintree
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

requirements/base.txt

@@ -1,4 +1,4 @@
-braintree==4.21.0
+braintree==4.28.0
 celery==5.3.1
 Django==5.0.6
 django-filter==22.1