[change] Remove 'accessibilityRelationship'; add 'dataSet'

Remove the 'accessibilityRelationship' prop which is not part of React Native.
The general approach to supporting ARIA-like accessibility APIs in React Native
needs revisiting and it will be easier to stop going down this path.

This patch also reverts the "unstable_ariaSet" change, and renames
"unstable_dataSet" to "dataSet". Avoiding another breaking change to
accessibility props will ease upgrading for now.

packages/docs/src/components/TextInput/TextInput.stories.mdx

@@ -14,7 +14,6 @@ some props are exclusive to or excluded from `multiline`.
 | Name                      | Type      | Default |
 | ------------------------- | --------- | ------- |
 | accessibilityLabel        | ?string   | |
-| accessibilityRelationship | ?Object   | |
 | accessibilityState        | ?Object   | |
 | autoCapitalize            | ?string   | 'sentences' |
 | autoComplete              | ?string   | 'on' |

packages/docs/src/components/View/View.stories.mdx

@@ -17,7 +17,6 @@ Please also refer to the in-depth "Accessibility" and "Style" guides.
 | ------------------------- | --------- | ------- |
 | accessibilityLabel        | ?string   | |
 | accessibilityLiveRegion   | ?string   | |
-| accessibilityRelationship | ?Object   | |
 | accessibilityRole         | ?string   | |
 | accessibilityState        | ?Object   | |
 | accessibilityValue        | ?Object   | |

packages/docs/src/guides/accessibility.stories.mdx

@@ -44,10 +44,6 @@ announce the change. (This is implemented using `aria-live`.)
 </Text>
 ```
 
-### accessibilityRelationship: ?object
-
-...
-
 ### accessibilityRole: ?string
 
 Indicates to assistive technologies how to describe the view, e.g., that it is a "button".