Improve heading order on Two-Factor Authentication settings page

When reviewing Delete OTP authenticator without affecting WebAuthn, I've noticed some accessibility violations:

1. Headings of the section are all h4, even though there is no h1, h2 or h3. They should all be changed to h2 with one of the gl-text-size-h* classes for styling purposes.
2. The page is missing h1 heading. The copy could be "Two-Factor Authentication" to mach the breadcrumbs.

Changes should be included in: https://gitlab.com/gitlab-org/gitlab/-/blob/22df0c739b39d635e779f94d50e1b6220cab7a0c/app/views/profiles/two_factor_auths/show.html.haml

I also recommend adding the accessibility check to the feature spec to ensure there's no regression in the future.

• here's how to add the test: https://docs.gitlab.com/ee/development/fe_guide/accessibility/automated_testing.html#how-to-add-accessibility-tests
• feature spec that should include the test: https://gitlab.com/gitlab-org/gitlab/-/blob/22df0c739b39d635e779f94d50e1b6220cab7a0c/spec/features/profiles/two_factor_auths_spec.rb

Results of accessibility audit scan: