Updated minimum php version from 7.3 to 7.4

[bookstack] / dev / api / responses / books-create.json

{
  "name": "My new book",
  "description": "This is a book created via the API",
  "created_by": 1,
  "updated_by": 1,
  "owned_by": 1,
  "slug": "my-new-book",
  "updated_at": "2020-01-12T14:05:11.000000Z",
  "created_at": "2020-01-12T14:05:11.000000Z",
  "id": 15
}