Enable `assumeImmutableResults` for creating a new epic

What does this MR do?

As a part of our Apollo 3 migration, we decided to set assumeImmutableResults to true on per-application basis. Thie MR changes the setting for the creating new epic Vue application

Screenshots or Screencasts (strongly suggested)

No visual changes

How to setup and validate locally (strongly suggested)

1. Open a list of group epics
2. Click "New Epic" button
3. Fill the fields and submit the form

There should be no errors in the process and the epic should be successfully created

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Related to #338564 (closed)