Mark legacy KeyboardEvent attributes as Obsolete

[dionrhys]

To reinforce that developers shouldn't make use of these legacy KeyboardEvent properties any more, this change marks them with the Obsolete attribute.

Sources:

• https://www.w3.org/TR/uievents/#legacy-key-attributes
• https://developer.mozilla.org/en-US/docs/Web/API/KeyboardEvent/charCode
• https://developer.mozilla.org/en-US/docs/Web/API/KeyboardEvent/keyCode
• https://developer.mozilla.org/en-US/docs/Web/API/KeyboardEvent/which