Report discarded notification observer as errors

[crazytonyli]

Fixes #20994.

This PR adds the discarded_notification_center_observer SwiftLint rule to this repo's SwiftLint configuration. All existing violations have been fixed (see the referenced PR in the issue). And from now all, new violations will be reported as errors.

As mentioned in the issue description. Observers not getting unregistered may cause problems because

• The block argument will not be released, which causes some objects not being released in some cases.
• The observer code is executed at a time that you don't expect them to execute.

Regression Notes

N/A

PR submission checklist: N/A

UI Changes testing checklist: N/A

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress Alpha by scanning the QR code below to install the corresponding build.

	App Name	WordPress Alpha
	Configuration	Release-Alpha
	Build Number	pr20973-bddf5db
	Version	22.8
	Bundle ID	org.wordpress.alpha
	Commit	bddf5db
	App Center Build	WPiOS - One-Offs #6479

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack Alpha by scanning the QR code below to install the corresponding build.

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr20973-bddf5db
	Version	22.8
	Bundle ID	com.jetpack.alpha
	Commit	bddf5db
	App Center Build	jetpack-installable-builds #5511

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[peril-wordpress-mobile]

	Warnings
⚠️	This PR is assigned to a milestone which is closing in less than 4 days Please, make sure to get it merged by then or assign it to a later expiring milestone

Generated by 🚫 dangerJS

[mokagio]

Excellent!