RFC: Officially drop node 8 support and remove it from testing in v2

[johno]

As part of the v2 release it is a good time to drop official support of node 8. There's no reason that MDX won't still continue to work with node 8, but we shouldn't need to test it anymore (it's EOL) and specify node 10+ in our changelog and migration guide.

[timneutkens]

Sounds great 👍💯