Ignore health_status column on epics

What does this MR do?

In !25995 (merged) we removed health_status column related code from epics. I, however, forgot to add ignore_column to the Epic model. This is needed so that we can drop the column from the database.

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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