Draft: Remove single member add path in the members API

What does this MR do and why?

• As per #352741 (closed) - removes the distinction in the members API between adding multiple or single users as members.

• no longer returns presented member object for a singular invite.

• note: placing back in draft due to customer use case - gitlab-com/gl-infra/production#6351 (closed)

Screenshots

before	after

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #352741 (closed)