Brevo: Rename SendinBlue to Brevo

- Replace "SendinBlue" with "Brevo"
  throughout the code.
- Maintain deprecated compatibility
  versions on the old names/URLs.
  (Split into separate commit
  to make renamed files more
  obvious.)
- Update docs to reflect change,
  provide migration advice.
- Update integration workflow.

pyproject.toml

@@ -66,6 +66,7 @@ dependencies = [
 # (For simplicity, requests is included in the base dependencies.)
 # (Do not use underscores in extra names: they get normalized to hyphens.)
 amazon-ses = ["boto3"]
+brevo = []
 mailersend = []
 mailgun = []
 mailjet = []