What problem does this pull request solve?

Trello card: https://trello.com/c/nsF4UoqP/3453-update-ttl-for-custom-statuspage-domain-to-86400

We've had this domain up for a while, so we can safely increase the TTL up to 24 hours.

Things to consider when reviewing

• Ensure that you consider the wider context.
• Does it work when run on your machine?
• Is it clear what the code is doing?
• Do the commit messages explain why the changes were made?
• Are there all the unit tests needed?
• Has all relevant documentation been updated?

Reminders

If you've made changes to the deployer role (files in modules/deployer-access):

• Remember to run make <environment> forms/account apply on the relevant environments (dev, staging, user-research, and/or prod)
• Check the #govuk-forms-deployment-notifications Slack channel to ensure the apply-forms-terraform-<environment> pipelines have run successfully