Check user signup date on public minutes tracking

What does this MR do?

This is a follow-up for the !27792 (merged) We don't want to limit CI Shared Runner minutes for public projects for users signed up before the policy change.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

N/A

Security

N/A

Relates to #210570 (closed)