Default the routing_api.enabled_api_endpoints to mtls

[mariash]

What is this change about?

Switch the default for routing_api.enabled_api_endpoints from "both" to "mtls" to prefer secure endpoints

What type of change is this?

• [x] [Breaking Change]: the change removes a feature or introduces a behavior change to core functionality (request routing, request logging)
• [ ] [Minor Feature/Improvement]: the change introduces a new feature or improvement that doesn't alter core behavior
• [ ] [Bug Fix]: the change addresses a defect

Backwards Compatibility

This should be backwards compatible since this property can be switched back to "both" by operator.

How should this be tested?

Deploy routing-release without setting routing_api.enabled_api_endpoints and see that the value of enabled_api_endpoints in routing-api job is set to "mtls"

PR Checklist

• [X] I have viewed, signed, and submitted the Contributor License Agreement.
• [X] I have made this pull request to the develop branch.
• [ ] I have run all the unit tests.
• [X] I have given thought to the backward-compatibility requirements of this change, and listed any mitigations above.
• [ ] (Optional) I have run Routing Acceptance Tests and Routing Smoke Tests.
• [ ] (Optional) I have run CF Acceptance Tests.