Mark "arcanist.projectinfo" as deprecated

Summary: Ref T7604. Mark the arcanist.projectinfo #conduit method as deprecated.

Test Plan: Went to the /config/issue/ page and saw the Deprecated Conduit Method "arcanist.projectinfo" In Use setup issue.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7604

Differential Revision: https://secure.phabricator.com/D12970