Move remaining gitlab.rb settings to underscored form

What does this MR do?

Move remaining gitlab.rb settings to underscored form as mentioned in #6873 (closed)

1. web_server
2. storage_check
3. runtime_dir
4. prometheus_monitoring
5. omnibus_gitconfig
6. manage_storage_directories
7. manage_accounts
8. high_availability
9. gitlab_ci
10. Different external_urls

Related issues

#6873 (closed), #7455 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened