Only allow multiple reviewers in paid tiers

Problem to solve

For best alignment with out tier strategy, multiple reviewers should be available only in Starter+ tiers.

Further information

Currently, multiple assignees is available in Starter+ tier https://docs.gitlab.com/ee/user/project/merge_requests/getting_started.html#multiple-assignees-starter

Proposal

Allow for a single reviewer in free tier and multiple reviewers in Starter+ tiers.