Rename the API group to API Platform to make remit clearer

The API group is recieing a lot of requests related to features in the API. The try and make the team's remit of fixing the underlying design and foundations of the API we're going to rename to API Platform.

This issue tracks the things we need to update:

• Team Slack channel
• API project epic automation
• API group label
• Handbook page
• Trige Ops
• Error budgets?
• API team member profiles
• ...other things

After the changes have been made, announce the new name across Engineering.