Resolve "Security Dashboard Component: Replace main-tag with article-tag"

What does this MR do?

This MR replaces the currently used <main> with an <article> to ensure component re-usability.

(Using a main tag within a vue-component introduces the possibility of having two or more main tags on a page, this causes accessibility issues)

Related discussion on MR that introduced the <main> is here: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/15050#note_210403348

Screenshots

No visible changes.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #14684 (closed)