Do not error when assigning issue to same epic

What does this MR do and why?

Do not error when assigning issue to same epic

When assigning an issue to the same epic we should just no-op and not error. Before this change the user would have seen an error that the issue is already assigned.

Changelog: fixed

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After
Screen_Recording_2024-06-06_at_20.20.52	Screen_Recording_2024-06-06_at_21.11.59
Screen Recording 2024-06-06 at 21.16.38.mov	Screen Recording 2024-06-06 at 21.12.54.mov

How to set up and validate locally

Pre-requirement

1. create an epic
2. create issue(s) and assign them to an epic

Swimlanes:

1. Go to Issue Boards, enable swimlane and add a new list with labels
2. move the issue around to the label, but in the same epic -> no error anymore

Bulk edit:

1. Go to issue list and bulk edit them and assign them to a new epic
2. Select the same issues again (and maybe more) and assign them to the same epic -> no errors anymore and all issues get assigned