Add feature specs for Upgrade Offer scenarios

What does this MR do?

This is the implementation MR for https://gitlab.com/gitlab-org/gitlab/-/issues/300145

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 these are developer facing changes.
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content