The adittion of multiple targets support requires that field be added to the notify call. You need to add an empty target line and that should fix things.
automation:
- alias: Startup Notification
trigger:
- platform: event
event_type: homeassistant_start
action:
- service: notify.pushover
data_template:
title: HA
message: 'HA is up and running..!'
target: ""