Fix: merge request editing page wrong translations

What does this MR do and why?

Root cause

The label was described as:

= form.label :assignee_id, issuable.allows_multiple_assignees? ? _('Assignees') : _('Assignee'), class: "col-12"

In the code snippet above, it works perfectly in English, but it would be strange in those languages that don't have plural forms such as Chinese.

_("Assignee") // Translates to `Assignees` but in Chinese, see the screenshot for more details.
// translates to plural form since the plural form did not exist and as result, the singular form is the plural form
// and it starts from 0.

In this MR the updates were mainly focused on removing the ambiguity we should avoid using _("Assignee") or _("Reviewer) as a label since they have plural forms.

/cc @prajnamas @orozot @qk44077907

Describe in detail what your merge request does and why.

Screenshots or screen recordings

Before fix

Form label

Dropdown header

After fix

Form label

Dropdown header

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

How to set up and validate locally

1. Boot your GDK
2. Go to any project and set the assignee to be exactly 1 assignee at most.
3. Go to the project's merge request and find an existing/create a new project to observe the changes.
4. Observe

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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