Keycodes: Do not test for `keyCode` in `isKeyboardEvent`

[swissspidy]

What problem does this address?

#43147 had an issue testing keyboard shortcuts because @testing-library/user-event removed support for keyCode in version 14, but isKeyboardEvent still tests for it.

What is your proposed solution?

If that util can be adjusted to test for key instead of keyCode, then workarounds like fireEvent.keyDown() are no longer needed.

[ciampo]

We could also rewrite the source components being tested to use code or key instead, since keyCode is effectively deprecated.

We'd just need to be careful with IME composition (see #45605)