[2315] Add migration to set decline by default

[elceebee]

Context

This is the second of three PRs to correct this bug related to decline_by_default. See the first PR here for context and to see what has been done already.

Changes proposed in this pull request

Job will set data to nil for about 40,109 applications. See blazer query.

I've tested it locally with 4000 application choices and it took 6.506ms, so I think this update will only take 65ms, no time at all.

Guidance to review

Will this touch 40,000 records? Am going to cause another BIG TOUCH incident? I don't think so because it doesn't execute active record callbacks, but if someone knows better, please speak up!

https://apidock.com/rails/ActiveRecord/Relation/update_all

Things to check

• If the code removes any existing feature flags, a data migration has also been added to delete the entry from the database
• This code does not rely on migrations in the same Pull Request
• If this code includes a migration adding or changing columns, it also backfills existing records for consistency
• If this code adds a column to the DB, decide whether it needs to be in analytics yml file or analytics blocklist
• API release notes have been updated if necessary
• If it adds a significant user-facing change, is it documented in the CHANGELOG?
• Required environment variables have been updated added to the Azure KeyVault
• Inform data insights team due to database changes
• Make sure all information from the Trello card is in here
• Rebased main
• Cleaned commit history
• Tested by running locally
• Add PR link to Trello card

[elceebee]

Bug: decline_by_default_at field is incorrectly being set