Fix Epic bulk updates leaking to other epics

What does this MR do?

When doing a bulk update on epics, the set of labels for the first selected epic are copied over to the rest of the epics.

This is because we compute the final list of labels and sets it to params and this params is the same one used for updating other epics.

Related to #338849 (closed) #327727 (closed)

How to setup and validate locally (strongly suggested)

1. Create epics with different sets of labels
2. Do a bulk update to add another label
3. On master, all the epics would end up having the same set of labels. With this MR, the bulk update behaves correctly.

Does this MR meet the acceptance criteria?

Conformity

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Availability and Testing

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