R1:0c967dd53d57

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