Prefer ActiveModel's attributes over Virtus

What does this MR do?

At GitLab we use Virtus sporadically.

The Piotr Solnica announced to discontinue this project (Twitter).

There are several alternatives available e.g. dry-types and we might end up using it if we decide to use some other gems from the dry-rb ecosystem as discussed in https://gitlab.com/gitlab-org/gitlab-ce/issues/56767.

In the meantime, ActiveModel borrowed its "type system" from ActiveRecord so we can define typed attributes via attribute :name, TYPE 🎉

Let's utilize this 💪

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• 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