Update MR form to use plural field names

Issue #244835 (closed)

What does this MR do?

Pluralize Assignee and Reviewer in MR form

Testing Case

1. Go to a Merge Request and click "edit".

In the free tier, it will display the singular wording. In the paid tier, it will display the plural wording, irrespective of the size.

Paid ( <= 1)	Paid ( > 1)	Free

Note

This change will also affect the Issues form page. Example:

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #244835 (closed)

Closes #244835 (closed)

app/views/shared/issuable/form/_metadata_issuable_assignee.html.haml

-= form.label :assignee_id, "Assignee", class: "col-form-label #{has_due_date ? "col-md-2 col-lg-4" : "col-sm-2"}"
+= form.label :assignee_id, issuable.allows_multiple_assignees? ? _('Assignees') : _('Assignee'), class: "col-form-label #{has_due_date ? "col-md-2 col-lg-4" : "col-sm-2"}"
 .col-sm-10{ class: ("col-md-8" if has_due_date) }
   .issuable-form-select-holder.selectbox
     - issuable.assignees.each do |assignee|