Mark issues in FixupBase abstract (#5033)

Since #5024 all fixups have an associated issue. Generally, it is
generally better to have an issue for every fixup so that things
can be mapped to repairs in Core easily. Let's mark the issues property
as abstract to indicate subclasses are required to implement it.

supervisor/resolution/fixups/base.py

@@ -58,9 +58,9 @@ class FixupBase(ABC, CoreSysAttributes):
         """Return a ContextType enum."""
 
     @property
+    @abstractmethod
     def issues(self) -> list[IssueType]:
         """Return a IssueType enum list."""
-        return []
 
     @property
     def auto(self) -> bool: