Adjust supported conversion types for epics

What does this MR do and why?

This change adds a new feature to control whether work items can be converted from one type to another. The code introduces a new setting called "supports_conversion" that determines if a work item type allows conversion to other types.

The main changes include:

• Adding a new method to check if conversion is supported (defaults to true for most types)
• Making Epic work items non-convertible by setting their conversion support to false
• Updating the conversion logic to return an empty list when conversion isn't supported
• Adding tests to verify that Epic work items cannot be converted and that the new conversion control works properly

In simple terms, this prevents certain types of work items (specifically Epics) from being changed into other types, giving administrators more control over how work items can be modified in the system.

References

`workItemType.supportedConversionTypes` should ... (#582916 - closed)

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