Refactor Pages deployment and artifact migrations

What does this MR do?

Relates to this follow-up issue.

In another MR, new rake tasks were introduced (gitlab:pages:deployments migrate tasks). As they were closely related to existing tasks (gitlab:artifacts tasks), in terms of workflow, that MR introduced some repetitions.

Consequently, this MR here does some refactoring on what's used by these two sets of tasks.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because this is developer-facing only (refactoring).
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability 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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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

Related to #327078 (closed)