Add warnings to JIRA auth provider that we only support JIRA 6

Summary: Ref T4289. Make it clear that this provider does not currently work with JIRA 5.

Test Plan: Viewed JIRA provider from `/auth/`, saw warnings.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T4289

Differential Revision: https://secure.phabricator.com/D7906