Handle Heroku maintenance status and unknown colors gracefully

[Montana]

Problem

The Heroku adapter only maps the green, yellow, and red status colors. Heroku's status page also reports a maintenance state (blue), and the adapter currently raises a raw KeyError when it encounters it:

Upcheck.for(:heroku).status
# => KeyError: key not found: "blue"

This has three consequences:

1. Uncaught crash during Heroku maintenance windows. Any blue system makes #status and #components raise KeyError, which is not an Upcheck::Error subclass — breaking the README's promise that "every failure raises a specific subclass of Upcheck::Error", and doing so precisely when a caller is most likely to be checking Heroku's status.
2. Provider#maintenance? is unreachable for Heroku. Nothing maps to "maintenance", so it can never return true.
3. An empty status array also raises (KeyError: key not found: nil, via max_by returning nil).

Fix

• Map blue → page status "maintenance" and component status "under_maintenance", with severity ranked between green and yellow (matching Statuspage's none < maintenance < minor < major convention, so a degraded system still wins over a maintenance window).
• Add a "Systems under maintenance" description.
• Raise Upcheck::ParseError (instead of KeyError) with a descriptive message if Heroku ever reports a color the adapter doesn't recognize, keeping the error-contract promise.
• Report "none" when the systems list is empty rather than raising.

Notes for reviewers

• I ranked maintenance below minor so that a simultaneous degradation takes precedence in the page-level status; happy to change if you'd prefer maintenance to rank higher.
• If you'd rather degrade gracefully on unknown colors (e.g., treat them as minor) instead of raising ParseError, that's a one-line change — raising felt more honest for a canonical-vocabulary API, but I don't feel strongly.

Testing

• New fixture spec/fixtures/heroku/current_status_maintenance.json plus a stub_heroku_body helper for anomalous inline payloads.
• 8 new examples covering: blue → "maintenance" status and "under_maintenance" components, yellow-beats-blue precedence, empty systems list, ParseError on unknown colors (both #status and #components), the maintenance description, and a Provider#maintenance? integration test.
• Full suite: 99 examples, 0 failures.