Adopt layout anchors

[frederoni]

Fixes #12144 and possibly #12143

Since we dropped support for iOS8, the layout anchor fluent API is now at our disposal. No more reaching out from MGLMapView to the containing app’s view controller to figure out how to layout the ornaments. This change also conforms to High Performance Auto Layout.

• Write a blurb in the changelog
• Adopt layout anchors for the diagnostic views

cc @fabian-guerra @1ec5 @julianrex @friedbunny

[julianrex]

How does this PR affect @1ec5 's Reinstall constraints only when needed PR ?

Does it supersede it?

[friedbunny]

This seems very nice, thanks @frederoni. I think there are a couple points to consider:

1. This removes MGLMapView.contentInset as a factor in these constraints, which I think is the right way to go, but it is a potentially significant change. Like @julianrex mentioned, we should consider whether or not Reinstall constraints only when needed #12144 (or a larger refactor) is needed.
2. We’ve had a variety of regressions and issues with these constraints over the years and I’d worry about that happening again — are we happy with the test coverage of constraints/ornament layout or does this change require more tests?

[1ec5]

How does this PR affect @1ec5 's Reinstall constraints only when needed PR ?

The iOS part of #12144 would be superseded if the layout anchors account for the automaticallyAdjustsScrollViewInsets and contentInsetAdjustmentBehavior properties. But that seems unlikely, given that we’re manually mimicking UIScrollView – other UIView subclasses don’t need that functionality.

If we feel good about this PR, I’d suggest making the same changes to the macOS implementation of MGLMapView, to fully obviate #12144.

[julianrex]

We should check this against #12053, #11491

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[friedbunny]

This is still planned, but needs robust tests/testing before we can accept it.

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[fabian-guerra]

Until #13911 lands will keep open this pr.

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[fabian-guerra]

This was addressed in #13911