Refactor Epic mutations base

What does this MR do?

Moves re-used epic field and arguments to the base mutation so they can be included in other mutations inheriting from Mutations::Epics::Base.

These changes would aim the work in progress here: !25987 (merged). And the downside is that some descriptions would be more generic.

Screenshots

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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done