Bump django-anymail from 14.0 to 15.0

Bumps [django-anymail](https://github.com/anymail/django-anymail) from 14.0 to 15.0.
- [Release notes](https://github.com/anymail/django-anymail/releases)
- [Changelog](https://github.com/anymail/django-anymail/blob/main/CHANGELOG.rst)
- [Commits](anymail/django-anymail@v14.0...v15.0)

---
updated-dependencies:
- dependency-name: django-anymail
  dependency-version: '15.0'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

requirements/production.txt

@@ -8,7 +8,7 @@ collectfasta==3.3.3  # https://github.com/jasongi/collectfasta (maintained fork
 
 # Django
 # ------------------------------------------------------------------------------
-django-anymail[mailgun]==14.0  # https://github.com/anymail/django-anymail
+django-anymail[mailgun]==15.0  # https://github.com/anymail/django-anymail
 
 # GCP Stuff
 # ------------------------------------------------------------------------------