Remove long time deprecated features from codebase (prepare for pipecd-v1 release)

[khanhtc1202]

What would you like to be added:

As preparation for PipeCD v1 release (scheduled in the end of this year), I want to clean up the codebase. As the result of this issue, features marked as deprecated would be:

• Deleted from the codebase (controlplane codebase)
• Has a clear decision that whether we bring that feature to the v1 codebase or not

This issue will be used to track the progress of deprecated features removal.

Note: The deprecated feature on pipedv0 could be remained in pipedv0 codebase, and will not be included in pipedv1.

Why is this needed:

[khanhtc1202]

Features/Configurations marked as deprecated:

• FileDB (Deprecate FileDB as PipeCD datastore #5652)
• EnableDefaultKubernetesCloudProvider config in piped (ref: https://github.com/pipe-cd/pipecd/blob/master/pkg/app/piped/cmd/piped/piped.go#L148)
• EventWatcher config via .pipe/ directory (ref: https://pipecd.dev/docs-v0.52.x/user-guide/event-watcher/#use-the-pipe-directory)
• SaveDeploymentMetadata in piped/service.proto (ref: https://github.com/pipe-cd/pipecd/blob/master/pkg/app/server/service/pipedservice/service.proto#L90C9-L90C31)
• Platform(XXX)Input.AutoRollback (ref: https://github.com/pipe-cd/pipecd/blob/master/pkg/config/application_ecs.go#L67-L68)
• App config NotificationMention.slack (ref: https://pipecd.dev/docs-v0.52.x/user-guide/configuration-reference/#notificationmention)
• Piped config CloudProviders/PlatformProvider (except for deployment metrics collector)
• Piped config Notifications.Receivers.Slack.OAuthToken (ref: https://pipecd.dev/docs-v0.52.x/user-guide/managing-piped/configuration-reference/#notificationreceiverslack)
• All KindPlatform in the application config other than KindApplication
• pipectl init command (ref: create pipectl init command #4677) -> Remove pipectl init command #5958
• TBD

Model / Internal:

• Model ArtifactVersion.Kind (ref: https://github.com/pipe-cd/pipecd/blob/master/pkg/model/common.proto#L95-L96)
• Model Deployment.Metadata (ref: https://github.com/pipe-cd/pipecd/blob/master/pkg/model/deployment.proto#L105-L106)
• Model PipelineStage.predefined (ref: https://github.com/pipe-cd/pipecd/blob/master/pkg/model/deployment.proto#L150-L151)
• Model PipelineStage.visible (ref: https://github.com/pipe-cd/pipecd/blob/master/pkg/model/deployment.proto#L153-L154) -> [pipedv1] Remove visible field from controller logic #5971
• TBD

[sagnik3788]

can i work on it EventWatcher config via .pipe/ directory ?
We are going to remove this fully from Piped, right?
@khanhtc1202

[khanhtc1202]

@sagnik3788 That feature is under discussion since our users currently prefer that over the in-application event watcher configuration, due to it compatibility with multiple application projects. So I think we would like to keep it for pipedv1

[shivansh-gohem]

Hi @khanhtc1202, I see the Kubernetes provider task is already completed.

I would like to pick up App config NotificationMention.slack instead. I can remove that field and clean up any related usage. Does that sound good?

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