- #7413 part 1 of 2
- made all `ModelMetadataProvider` and `ObjectModelValidator`-specific code conditional
- fortunately, `MvcOptions` easy to get; affected code is primarily `internal` or pub-`Internal`
- remove unnecessary `ModelMetadataProvider` use in `ApiBehaviorApplicationModelProvider`
- run integration and functional tests with `CompatibilityVersion.Version_2_1`
- functional test change depends on @javiercn's recent #7541 fix
- remove test code now redundantly turning compatibility switches on
nits:
- correct spelling errors in `CompatibilitySwitch`
- take VS suggestions, mostly in test code
- rename methods in `ControllerBinderDelegateProviderTest` to match current API
- slightly refactor in `ApiBehaviorApplicationModelProvider`
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