Restore and fix tvOS checks and annotations and remove PlatformVersion

[akoeplinger]

Follow-up to #6237, some of the tvOS version checks got lost there since PlatformVersion.IsAtLeast() which checks both iOS and tvOS was replaced by just checking for iOS.

Fixed that and also removed the PlatformVersion class (follow-up to #5860)

[akoeplinger]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[mattleibow]

Just 1 Q on something that seems to violated compile rules before...

[mattleibow]

I notice a version change... Is this a bug in the macios repo because it should not have compiled?

[akoeplinger]

looks like UINavigationBarAppearance was introduced in iOS13, so looks like a MAUI issue and this should've been called on iOS13?

[akoeplinger]

The code was introduced in #3332 so definitely intended for iOS 15, hence the method name.

I'm not an expert here so I'd keep it as is for now and maybe file an issue for someone to consider lowering this to iOS13?

[buyaa-n]

JFYI: The warnings found were all for version 13:

'UINavigationBar.StandardAppearance.set' is   only supported on: 'ios' 13.0 and later, 'maccatalyst' 13.0 and later, 'tvos'   13.0 and later.
'UINavigationBarAppearance' is only   supported on: 'ios' 13.0 and later, 'maccatalyst' 13.0 and later, 'tvos' 13.0   and later.
'UIBarAppearance.ConfigureWithOpaqueBackground()'   is only supported on: 'ios' 13.0 and later, 'maccatalyst' 13.0 and later,   'tvos' 13.0 and later.
'UIBarAppearance.BackgroundColor' is only   supported on: 'ios' 13.0 and later, 'maccatalyst' 13.0 and later, 'tvos' 13.0   and later.
'UINavigationBarAppearance.TitleTextAttributes'   is only supported on: 'ios' 13.0 and later, 'maccatalyst' 13.0 and later,   'tvos' 13.0 and later.
'UINavigationBar.ScrollEdgeAppearance' is   only supported on: 'ios' 13.0 and later, 'maccatalyst' 13.0 and later, 'tvos'   13.0 and later.