Rename notification event model

[nghialv]

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

NONE

[pipecd-bot]

Some issues were detected while linting go source files in your changes.

[nakabonne]

Seems that this model is used by only Piped and no communication using it exists. LGTM except for my comment 👍

[nakabonne]

/lgtm

[pipecd-bot]

Code coverage for golang is 33.79%. This pull request decreases coverage by -0.02%.

File	Function	Base	Head	Diff
pkg/model/notificationevent.go	NotificationEvent.Group	--	66.67%	+66.67%
pkg/model/notificationevent.go	NotificationEventDeploymentTriggered.GetAppName	--	100.00%	+100.00%
pkg/model/notificationevent.go	NotificationEventDeploymentPlanned.GetAppName	--	0.00%	+0.00%
pkg/model/notificationevent.go	NotificationEventDeploymentApproved.GetAppName	--	0.00%	+0.00%
pkg/model/notificationevent.go	NotificationEventDeploymentRollingBack.GetAppName	--	0.00%	+0.00%
pkg/model/notificationevent.go	NotificationEventDeploymentSucceeded.GetAppName	--	0.00%	+0.00%
pkg/model/notificationevent.go	NotificationEventDeploymentFailed.GetAppName	--	0.00%	+0.00%
pkg/model/notificationevent.go	NotificationEventApplicationSynced.GetAppName	--	0.00%	+0.00%
pkg/model/notificationevent.go	NotificationEventApplicationOutOfSync.GetAppName	--	0.00%	+0.00%
pkg/app/piped/logpersister/stagelogpersister.go	stageLogPersister.flushFromLastCheckpoint	61.11%	50.00%	-11.11%
pkg/model/event.go	Event.Group	66.67%	--	-66.67%
pkg/model/event.go	EventDeploymentTriggered.GetAppName	100.00%	--	-100.00%
pkg/model/event.go	EventDeploymentPlanned.GetAppName	0.00%	--	+-0.00%
pkg/model/event.go	EventDeploymentApproved.GetAppName	0.00%	--	+-0.00%
pkg/model/event.go	EventDeploymentRollingBack.GetAppName	0.00%	--	+-0.00%
pkg/model/event.go	EventDeploymentSucceeded.GetAppName	0.00%	--	+-0.00%
pkg/model/event.go	EventDeploymentFailed.GetAppName	0.00%	--	+-0.00%
pkg/model/event.go	EventApplicationSynced.GetAppName	0.00%	--	+-0.00%
pkg/model/event.go	EventApplicationOutOfSync.GetAppName	0.00%	--	+-0.00%

[khanhtc1202]

🚀
/approve

[pipecd-bot]

This pull request is APPROVED by khanhtc1202.

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