Add issue template for Open Source migrations

What does this MR do?

This MR adds an issue template for open source migrations. It's based off of the KDE migration issue: #24900 (moved)

Does this MR meet the acceptance criteria?

Conformity

I think most of these are N/A since it's an issue template.

• Changelog entry -- don't think this is relevant since it's an issue template.
• Documentation (if required)
• Code review guidelines - N/A
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

No risks other than potentially confusing people.