MINOR: Update connector status metric description to include 'stopped' as a potential value

[yashmayya]

• KIP-875 introduced a new STOPPED state for connectors (also see KAFKA-14783 (KIP-875): New STOPPED state for connectors #13424).
• This new state is a potential value for the connector status metric that is computed here and hence needs to be added to the metric's description.
• Note that this doesn't need to be done for the task status metric since tasks are shutdown when a connector is stopped (i.e. a connector can be in a stopped state, but a task cannot).

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)

[mimaison]

LGTM, thanks!