Add dont-land-on-v6.x and v8.x labels for ES Modules PRs

[gibfahn]

I'm not sure if this is possible, but if someone adds the ES Modules label, the bot should add the dont-land-on labels.

e.g. nodejs/node#17786

cc/ @MylesBorins

[MylesBorins]

should they be don't land for 8.x?

[gibfahn]

Sorry, bad grammar, I meant dont-land-on for v6.x and v8.x

i.e. dont-land-on-v{6,8}.x

[joyeecheung]

The bot should be able to add the ES Modules label by itself anyway.

[richardlau]

This can be closed as Node.js 6 and 8 are End-of-Life.