Fix spec violations in ee/spec/frontend/boards/milestone_select_spec.js

Problem description

The spec file ee/spec/frontend/boards/milestone_select_spec.js contains technical debt and/or antipatterns that need to be fixed. This is a prerequisite for updating our codebase to Vue3.

Proposal

The following issues have been identified in this spec file:

• overriding method behavior

Please see the related epic for more details on how these violations can be addressed.