feat(multi-connections): add support for destroy integration customer mutation (#6018)

## Context
Removing an integration customer connection was only possible as a
side-effect of a customer update. As part of multi-connections, we need
a dedicated action to delete a specific connection.

## Description
Add a `destroyIntegrationCustomer` GraphQL mutation backed by
`IntegrationCustomers::DestroyService`. Hard delete is kept for this
first version, consistent with the current behavior.