Remove MDN Newsletter Subscriptions from Database

[robhudson]

The MDN team has discontinued the MDN newsletter. We need to delete all records in the newsletters table where the newsletter name is 'mdnplus'. There have been no prior campaigns, so only the subscription entries exist without any associated engagement data.

Tasks:

• Create an Alembic migration to remove rows with name='mdnplus' from the newsletters table.
• Test the migration in a staging environment to ensure it functions correctly.
• Deploy the migration to production after successful testing.