revert ghaction-import-gpg version back to 5

[fantapop]

It seems like there is some approval process for using a later version of this and version 6 is not allowed. For now we revert back to version 5 of this action.

Commit checklist

• [ ] Changelog
• [ ] Doc gen (make generate)
• [ ] Integration test(s)
• [ ] Acceptance test(s)
• [ ] Example(s)