Replace all ContextManager sharedInstance() usages with shared

[mokagio]

Apart from being more Swift-y, this will also allow us to differentiate between a shared instance for the Objective-C layer (CoreDataStack) and one for the Swift layer (CoreDataStackSwift).

This differentiation will be valuable if/when we'll replace ContextManager references with CoreDataStack/CoreDataStackSwift so that WordPressDataObjC can refer to the functionality without knowing about ContextManager, which is defined in the Swift WordPressData layer.

For example, the Objective-C AbstractPost implementation references the Swift ContextManager implementation

WordPress-iOS/WordPress/Classes/Models/AbstractPost.m

Line 32 in b8f184b

[[ContextManager sharedInstance] saveContext:self.managedObjectContext];

I'm doing this change in a dedicated PR because there are 100+ usages of sharedInstance and because I don't know yet where the work of replacing ContextManager access with CoreDataStack in the Objective-C layer will bring me...

Testing

This is just a simple swap, given that shared wraps sharedInstance(). If the code compiles, we should be good.

I still gave the app a run on my device and loaded a few screens just to ensure it runs okay.

---

Part of #24165

[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	pr24187-59884f7
	Version	25.8
	Bundle ID	org.wordpress.alpha
	Commit	59884f7
	App Center Build	WPiOS - One-Offs #11631

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	pr24187-59884f7
	Version	25.8
	Bundle ID	com.jetpack.alpha
	Commit	59884f7
	App Center Build	jetpack-installable-builds #10662

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