Widgets: Provide mandatory iOS 17 SDK Support

[staskus]

Tasks

• [x] https://github.com/wordpress-mobile/WordPress-iOS/pull/21705
• [x] Update small Home screen widget layout to support removable background and appear on iPadOS
• [x] Consider updating Lock Screen Widget layout to appear without background as most Lock Screen Widgets do
• [ ] Support margins and allow at least some of the widgets to appear in StandBy mode
• [ ] https://github.com/wordpress-mobile/WordPress-iOS/issues/21816

Description

After building the app with Xcode 15, Home and Lock screen widgets are no longer working and show please adopt containerBackground API message:

This is related to a new Removable background introduced at WWDC2023. More context:

• https://developer.apple.com/documentation/widgetkit/preparing-widgets-for-additional-contexts-and-appearances
• https://www.optimistic-closures.com/adjusting-your-widgets-containerbackground/
• https://nemecek.be/blog/192/hotfixing-widgets-for-ios-17-containerbackground-padding

Expected behavior

Widgets should support new APIs and this message should not be shown

Steps to reproduce the behavior

1. Build Jetpack app with Xcode 15
2. Try to add a widget

Tested on [device], iOS [version], Jetpack iOS / WordPress iOS [version]

[staskus]

In order to remove this message we need to support containerBackground API. The best way is to create an extension so both iOS17 and <=iOS16 would be supported:

extension View {
    func widgetBackground(_ backgroundView: some View) -> some View {
        if #available(iOSApplicationExtension 17.0, *) {
            return containerBackground(for: .widget) {
                backgroundView
            }
        } else {
            return background(backgroundView)
        }
    }
}

and attach it to the end of the view. This makes the background removable. Most notably, on iPadOS Lock Screen.

Issue

We use AccessoryWidgetBackground for our background. I couldn't find a way to integrate to containerBackground API, since SwiftUI requires to present it within ZStack. There were questions related to this on Apple forums. Likely that there's a solution I couldn't identify.