Create a new BR method for ACME dns-01

[CBonnell]

This topic was originally raised on the 2026-04-09 servercert-wg call and discussed on the 2026-04-16 validation-sc meeting.

There was agreement it would be useful to do, but not urgent. Some points raised:

1. There will need to be an effective to give time for CAs to transition from method 7 to the new method
2. It might be possible to add a restriction to method 7 to disallow it for use with dns-01 (i.e., "_acme-challenge" cannot be the subdomain)
3. It might also be possible to create the method dns-01 as a sub-method of method 7.

[github-actions]

This issue was created based on:

• TLS BR Version 2.2.6
• EVG Version 2.0.2