Refactor the logic to find accounts in the specified event

[ono-max]

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]

Code coverage for golang is 32.06%. This pull request increases coverage by 0.01%.

File	Function	Base	Head	Diff
pkg/config/deployment.go	DeploymentNotification.FindSlackAccounts	--	0.00%	+0.00%
pkg/app/piped/controller/planner.go	planner.getMentionedAccounts	0.00%	0.00%	+0.00%
pkg/app/piped/controller/scheduler.go	scheduler.getMentionedAccounts	0.00%	0.00%	+0.00%
pkg/app/piped/trigger/deployment.go	Trigger.triggerDeployment	0.00%	0.00%	+0.00%

[nghialv]

/lgtm

[nakabonne]

👍
/approve

[pipecd-bot]

This pull request is APPROVED by nakabonne.

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