added last successful to the backup envelope of linode

Author: hzoppetti

openapi.yaml

@@ -15105,6 +15105,12 @@ components:
                   - W18
                   - W20
                   - W22
+            last_successful:
+              type: string
+              format: date-time
+              description: The last successful backup date. 'None' if there was no previous backup.
+              readOnly: true
+              example: '2018-01-01T00:01:01'
         watchdog_enabled:
           type: boolean
           description: >