Update domain verification page route

What does this MR do and why?

Change domain verification route

• before:

group_settings_domain_verification_path(group)

• after:

group_settings_domain_verification_index_path(group)

Splited from MR !115766 (closed)

/cc @daveliu

Describe in detail what your merge request does and why.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

1. simulate the SaaS locally:

   export GITLAB_SIMULATE_SAAS=1

2. Upload Premium license to enable domain_verification feature

3. Visit domain verification page http://localhost:3000/groups/flightjs/-/settings/domain_verification

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.