Fix security scheme issues in Delivery API OpenAPI spec

[kjac]

Prerequisites

• I have added steps to test this contribution in the description below

If there's an existing issue for this PR then this fixes #17300

Description

The Delivery API OpenAPI spec wrongfully includes the back-office security scheme, because this is added to all OpenAPI specs by default.

This PR ensures that the Delivery API OpenAPI spec has no security scheme by default, but still allows for opting into the Umbraco member security scheme as per the docs.

The Umbraco member security scheme has been renamed from Umbraco Member to UmbracoMember, as whitespaces are not allowed in OpenAPI security scheme identifiers.

Testing this PR

1. Verify that the Delivery API OpenAPI spec does not include any security schemes by default.
2. Verify that it's possible to opt into the Umbraco member security scheme.
3. For both of the above, use an OpenAPI schema validator (for example https://editor.swagger.io/ or https://validator.swagger.io/) to ensure the validity of the spec.
4. Verify that this PR has no impact on the Management API OpenAPI spec.

[elit0451]

Works like a charm 💪 Merging 🔥