API support for `not` filter for Merge Requests

Problem to solve

not filtering was introduced for the Issues API:

• https://gitlab.com/gitlab-org/gitlab/blob/cf07c4456ee699db6f61064d07373fd42fcf7ede/lib/api/issues.rb#L38-40

This would also be super useful for Merge Requests

Intended users

• Sasha (Software Developer)

Proposal

Same support for filtering without labels, etc, for Merge Requests

Permissions and Security

Same permissions for viewing Merge Requests as dictated by our usual policies

Links / references